Combining PDF files in Preview, with nothing else installed

Preview combines PDF files, and it does it well enough that installing anything else is usually unnecessary. What trips people up is not the merge itself. It is that Preview has no Save step and no new document: the combined PDF is one of the files that went into it, changed in place, and the other original may already have been altered too.

That single property explains most of what goes wrong with this task. It also means the useful version of the instructions has a step before the first step, and Apple's own documentation says so plainly. Everything else is detail about which of three insertion methods suits which situation.

Duplicate first, because Preview does not ask

Apple's documented procedure for combining PDFs is short. Open the PDFs to combine. In each one, choose View > Thumbnails to show the page thumbnails in the sidebar. Then drag the thumbnails to add from the sidebar of one PDF to the sidebar of the other. Pages can be added at the end of the document or between existing pages, and dragging thumbnails reorders them.

For selecting pages that are not next to each other, Apple documents Command-clicking each one.

The step that belongs first is in the same documentation, stated as a condition rather than an instruction: "Changes you make to a Preview document are saved automatically. If you want to keep the original PDFs (in addition to the new combined one), choose File > Duplicate to create a copy of each PDF before combining them."

Read that as an instruction rather than a note. The document that receives the dragged pages becomes the combined file, and it is one of the originals. There is no untitled third document. Nothing prompts for a filename. Anyone who expected to end up with three files, two originals and one merged, ends up with two.

The source document is affected as well. Dragging a thumbnail out of one sidebar and into another moves the page rather than copying it, which is why the source PDF comes out shorter than it went in. Duplicating both files first covers both halves of that.

When it has already happened: Revert To

Preview supports the system's document versions, which is the recovery route for a merge that went into the wrong file. Apple documents its File > Revert To menu with entries for Previous Save, Last Saved and Last Opened, each labelled with a date and time, plus Browse All Versions for the full timeline.

Browse All Versions shows the current version against a stack of earlier ones. Clicking Restore replaces the current version, which is worth knowing before clicking it. Holding Option turns that button into Restore a Copy, which opens the earlier version in a new window instead of overwriting anything. When the question is which of several states was correct, Restore a Copy is the version to use, because a wrong guess costs nothing.

This is also the answer to the most common complaint about this task, which is that an original PDF now has extra pages in it. The original is not lost. It is one menu away, provided the check happens before the file has been saved over many more times.

Three ways to bring pages in

The drag between sidebars is the documented method, but it is not the only one, and the other two are better for specific shapes of the job.

Dragging thumbnails between two open PDFs

Best for picking specific pages out of one document. Both files have to be open, and both need View > Thumbnails turned on. Apple documents View > Contact Sheet as an alternative view for the same operations, which is easier when there are many pages to scan through.

Dragging a whole file onto the sidebar

Apple documents this as the method for adding an entire PDF: "drag the PDF icon from the Finder to the sidebar of an open document." One drag, all pages, no need to open the second file at all. For assembling a report out of several complete documents, this is the fastest route in Preview.

Edit > Insert > Page from File

Apple documents selecting the page that should come before the new one, then choosing Edit > Insert, then Page from File. The same submenu has Blank Page, which is occasionally what is actually wanted, for example as a separator before double sided printing.

What you are doing	Best method	Notes
Taking a few pages from one PDF	Drag thumbnails between sidebars	Command-click for pages that are not adjacent
Appending a whole document	Drag the PDF icon from the Finder to the sidebar	The second file does not need to be open
Inserting at an exact position	Select the preceding page, then Edit > Insert > Page from File	Also offers Blank Page
Removing pages after merging	View > Thumbnails, select, press Delete	Documented alongside the insert commands
Reordering the result	Drag thumbnails in the sidebar	Works in Contact Sheet view as well

Deleting is worth noting because merges usually need it. Apple documents choosing View > Thumbnails or View > Contact Sheet, selecting the page or pages, then pressing the Delete key. It also documents the side effect, which is easy to miss on a marked up document: when a page is deleted from a PDF, all the annotations on that page are removed with it. Highlights and notes are attached to pages, not to the file, so trimming a merged document throws away comments along with the paper.

There is a fourth insertion route that no list of steps usually mentions, and it is the only one in Preview that creates a new file. Apple documents it in a single line: a new PDF can be created by dragging thumbnails from a PDF sidebar to the desktop. Select the pages wanted, drag them out of the window, and a new PDF appears containing only those pages, with the source document left as it was. For pulling a five page extract out of a hundred page report, that is both faster and safer than merging into a duplicate and deleting the rest.

One position rule is worth committing to memory, because getting it wrong means undoing the insert. Apple documents that Edit > Insert adds the new page after the currently selected one, not before it. Select the page that should precede the insertion, not the page that should follow it.

The Finder route, which does not touch the originals

For more than two files, there is a documented method that skips Preview entirely and produces a genuinely new document.

Select the files in the Finder, Control-click them, then choose Quick Actions > Create PDF. Apple documents the result: "The file is created automatically with a name similar to the first file you selected." The ordering rule is documented too, and it is the part people miss: "The files appear in the PDF in the same order that you select them." Selection order, not name order and not the order they appear in the window.

The same command has two other homes. Apple documents a Create PDF button at the bottom of the Preview pane of a Finder window, reachable with View > Show Preview if the pane is hidden. Quick Actions including rotate, markup and create a PDF are also available directly from the preview in Gallery view.

The important difference from the Preview method is that this one leaves every input file alone. Nothing is edited in place, nothing needs duplicating first, and there is no source document that comes out shorter. For assembling scanned pages or a set of invoices, this is the route with fewer ways to go wrong. The trade is control: pages arrive whole, in selection order, and any trimming happens afterwards in Preview.

Selecting files in one window and running an action on them in another is the shape of most everyday file work on a Mac, which is why a file manager that keeps image and PDF jobs in the same window treats merging as a built in operation rather than a trip to another app.

When the merge becomes a weekly job

Both routes above are hand work, and hand work is where the duplicate step gets skipped. macOS ships two ways to turn the same job into something repeatable, and neither needs anything installed.

Automator is the older of the two and the better fit for this particular task, because its supplied library is unusually strong on PDFs. Alongside actions for splitting, watermarking, encrypting, renaming and extracting text, it includes one called Combine PDF Pages, which takes a list of PDF files and produces a single document. A workflow built around it decides where the output is written as part of the workflow, so the question of which original gets modified does not arise. Apple also documents saving a workflow as a Quick Action, which then appears in Finder windows, the Services menu and the Quick Actions menu. A merge tuned to a particular job therefore ends up in the same Control-click menu as the built in Create PDF, one step from the files.

Both automation systems have a command line entry point, which matters if the files arrive from a script rather than from a person. The automator command runs a saved workflow and takes its input with -i, including from standard input, so a list of paths produced by something else can be piped straight into a merge. The shortcuts command does the equivalent for the Shortcuts app, with run taking --input-path and --output-path, so a shortcut that assembles a PDF can be called by name from a shell.

Worth knowing what is not there: sips, the image processing tool that ships with macOS and handles conversion, resizing and colour profiles, has no PDF merge. The temptation to reach for it and then conclude that the terminal cannot do this job is a common dead end. The route is a saved workflow or shortcut, called from the shell, rather than a single built in command.

After the merge: size, and locking the result

Merged PDFs get large, especially from scans, and Preview has a documented answer. Choose File > Export, then pick Reduce File Size from the Quartz Filter menu, then Save, then choose whether to replace the existing file or save under a new name. Apple's warning on that page is worth quoting as written: "When compressed, the PDF may be of lower quality than the original."

The same export dialog offers finer control through three documented checkboxes: Create Linearized PDF, which optimises the file for the web, Optimize images for screen, and Save images as JPEG. For a document that has to stay legible, those are preferable to the blanket filter, because the effect of each one is predictable.

There is one more step that prevents the whole problem from recurring. Apple documents locking a PDF in Preview "so that it can't be edited or deleted," with the consequence that others "can change the document's contents only by creating a duplicate of the document and editing the duplicate." Unlocking is done from the arrow next to the document's name in the title bar, by deselecting the Locked checkbox.

For a finished, distributed PDF, locking it turns Preview's automatic saving from a liability into a non issue. Nothing can be dragged into it by accident.

What to change first

Choose File > Duplicate before dragging a single thumbnail, every time, because Preview will not ask and the receiving document is an original. For three files or more, use the Finder route instead: select them in the order they should appear, Control-click, and choose Quick Actions > Create PDF, which leaves every input untouched.

If merging PDFs is a weekly job rather than an occasional one, the trip between the window holding the files and the app doing the work is the part worth removing. Atriens has the PDF and image operations built into the file window, and the comparison page sets out what that covers.