Open both PDFs, turn on View then Thumbnails in each, and drag the thumbnails from one sidebar into the other. Or drag a whole PDF from the Finder onto an open document's sidebar.
Preview saves as you go. The document you dragged into is changed on disk, not copied. Apple's page says to use File then Duplicate first if you want to keep the originals, and it is the first thing on the page for a reason.
This is one of the genuinely good things about a Mac: a capability most operating systems make you install something for is sitting in an application you already use to look at the file. It is also one of the few places where the default behaviour can lose you work.
The drag
Open both PDFs in Preview
Two windows, or two tabs. Either works.
View, then Thumbnails, in each one
The sidebar is what you drag between. Without it there is nothing to drop onto.
Drag the thumbnails across
Command-click to pick several that are not next to each other. Drop between two existing pages to insert rather than append.
Or drag the file itself
Dragging a PDF's icon from the Finder onto an open document's sidebar adds the whole thing, which is quicker when you want all of it.
Apple's page for this is worth reading once, partly because the thumbnail sidebar is easy to miss and partly because of what it says at the top.
The warning at the top of Apple's own page
The fix takes one keystroke. Open the document, press Command-D for File then Duplicate, and merge into the copy. Or drag both originals into a third, new document rather than into one of them.
The Finder route, which touches nothing
There is a second way that avoids the problem entirely, and a lot of Mac users have never found it. Select several PDFs in the Finder, right-click, go to Quick Actions, and choose Create PDF. A new document appears next to them and the originals are untouched.
Makes a new file and leaves the originals alone.
Takes images as well as PDFs, in one go.
Orders by how the files are selected, not how they are named.
No control over page order beyond that.
Full control: insert anywhere, reorder by dragging.
Changes the document you drag into, unless you duplicate first.
Command-click to move several pages at once.
The only built-in route if you need a specific order.
Getting the order right
The Finder route orders pages by the order you selected the files, which is not the order they appear in the window and not alphabetical. Select them one at a time with Command held down, in the order you want, and it behaves. Select them with a rubber band and you will be reordering afterwards.
Preview's sidebar is the reliable answer for anything where order matters: drag the thumbnails into place, and what you see is what the document is.
What survives the merge
Putting two documents together is not only about pages. Each one may carry things that live at the document level rather than the page level, and those are where a merge quietly loses something.
Every page, at its own size. A merged document can have mixed page sizes and that is fine.
The text, still selectable and searchable.
Images, at their original resolution.
Page rotation, so a sideways scan stays sideways.
Bookmarks, unless the tool goes out of its way to carry them.
Form fields, which can collide when two documents use the same field names.
Links between pages, which point at page numbers that have moved.
Document metadata: the merged file takes one document's title, or none.
Form fields are the one that catches people. Two copies of the same form, merged, give you a document where typing in one field fills in the other, because as far as the file is concerned they are the same field. Flattening each form before merging avoids it, and flattening is a one-step job.
Mixed page sizes are worth a look too. A merged document can happily hold an A4 report and a Letter appendix, and it will print with one of them shrunk or shifted. Putting them on one size first is the fix if the result is going on paper.
When the browser is simply easier
If you are merging half a dozen files, want a specific order, and would rather not think about which document is about to be modified, the merge tool here shows every file in a list you can drag, builds a new document, and changes nothing on disk. It runs in Safari, so nothing is uploaded and nothing is installed.
Merge PDFs in the browser
Drag to order, build a new file, leave the originals alone.
Common questions
The drag, the save, the order, and the route that leaves your files alone.
01How do I combine two PDFs in Preview?
Open both, choose View then Thumbnails in each, and drag the thumbnails from one sidebar to the other. To add an entire document, drag its icon from the Finder onto an open document's sidebar.
02Does Preview overwrite the original when I merge?
Yes. Preview saves changes automatically, so the document you drag pages into is modified on disk. Apple's own instructions say to use File then Duplicate first if you want to keep the originals.
03Is there a way to merge without changing either file?
Select the PDFs in the Finder, right-click, then Quick Actions, then Create PDF. That makes a new document beside them and leaves both originals alone.
04Why are my pages in the wrong order?
The Finder route uses the order you clicked the files in, not the order they are listed or their names. Command-click them one at a time in the order you want, or use Preview's sidebar where you can drag pages into place.
05Can I combine images and PDFs together?
Yes. Finder's Create PDF takes images as well, and Preview will accept an image dragged into a PDF's thumbnail sidebar. Each image becomes a page.
06What if I already saved over my original?
Preview keeps versions. Open the file, choose File then Revert To, and Browse All Versions to step back to how it was before the merge. That only helps on the Mac where the change happened.