iOS reads, annotates, rotates, crops and inserts blank pages. It has no merge.
Three things work: a Shortcut you build once, an app from the store, or a browser tool in Safari. The last needs nothing installed and, if it runs locally, sends nothing anywhere.
This comes up most often away from a desk: two documents arrive by email, somebody wants one file, and the only machine to hand is a phone.
What iOS already does
More than its reputation. Notes in particular is a capable PDF tool that almost nobody uses as one: Apple's page for it covers attaching a PDF to a note, viewing it at several sizes, annotating it with Markup, and editing individual pages.
Markup is the other half of it: open a PDF from Files or Mail, tap the pen, and you can sign, highlight and draw. For returning a signed form that is the whole job, and the file never goes anywhere.
The Shortcuts route
Shortcuts is built in and it can be made to do this. The shape of the shortcut is: take the files as input, then a Make PDF action, then save or share the result. Built once it lives in the share sheet, so merging afterwards is selecting two files and tapping Share.
It is worth building anyway if this is a regular job. A shortcut that lives in the share sheet turns merging into two taps from wherever the files are, and Shortcuts can chain the rest of it: rename the result, drop it in a particular folder, or attach it to a message. That is time a web page cannot save you.
The route with nothing to install
Safari on iOS runs the same JavaScript as Safari on a Mac, which means a tool that does the work in the page works on the phone. The merge tool here is one: pick the files from Files, iCloud Drive or Dropbox through the standard picker, drag them into order, and the finished document saves back to Files.
Open the tool in Safari
Nothing to install and nothing to sign in to.
Tap to add files
The iOS picker offers Files, iCloud Drive and anything else you have connected.
Drag them into order
The list reorders by dragging, which is easier on a touchscreen than anywhere else.
Save back to Files
The download goes to Files, in the location you choose.
The thing worth checking with any web tool on a phone is whether the file is being uploaded. On a mobile connection that is slow, on a shared network it is worth thinking about, and for a bank statement or a signed contract it is the whole question. Tools that run in the page do not upload; tools that do the work on a server have to.
There is a practical tell. Put the phone into aeroplane mode after the page has loaded and try the job. A tool doing the work locally carries on as though nothing happened. A tool sending the file somewhere stops, usually with a spinner that never finishes.
Where the second PDF usually comes from
Half the time the reason somebody wants to merge on a phone is that one of the two documents was made on it. The camera is a good scanner and both routes to it are built in.
Notes
New note, the camera button, Scan Documents. It finds the edges, corrects the perspective and takes several pages in a row. Share the note's attachment as a PDF when you are done.
Files
The three dots in the top right, then Scan Documents. This one saves straight into a folder as a PDF, which is usually what you want if it is going to be merged with something else.
Check the order before you leave
Both let you retake a page, and reordering afterwards is more work than getting it right at the time.
Then merge
Two PDFs on the phone, one of them fresh from the camera, is exactly the case the rest of this page is about.
A scan made this way is a picture of a page, so there is no text in it to search until something reads it. If the merged document needs to be searchable, running it through OCR afterwards is the step that puts the text in.
Which one to use
Signing or annotating: Markup, no question.
Rotating or cropping one page: Notes.
Merging two or three files, once: Safari and a browser tool.
Merging often: a Shortcut, built once and kept in the share sheet.
A long document that needs reordering page by page.
Anything where you want to see the whole thing at once.
Filling a complicated form, which is miserable on a phone screen.
Batch work across many files.
Merge PDFs in Safari
Pick the files, drag them into order, save back to Files. Nothing installed and nothing uploaded.
Common questions
What iOS gives you, what it does not, and the route with nothing to install.
01Can the Files app merge two PDFs?
No. Files will view, rename, move and share a PDF, and it has no merge. Selecting several images gives you an option to make a PDF from them, which is a different thing.
02Can I merge PDFs in the Notes app?
Not into one document. Notes will hold several PDFs in the same note and let you annotate, rotate, crop and insert blank pages in each, but they remain separate attachments.
03Does Shortcuts have a merge action?
There is no action called Merge PDFs. The usual approach is a Make PDF action taking several files as input, which has behaved differently across iOS versions, so test it before trusting it.
04Will a browser tool work on an iPhone?
Yes, if it is written to run in the page. Safari on iOS runs the same JavaScript engine as on a Mac, so the work happens on the phone. The tools here are built that way and do not upload anything.
05Where does the merged file go?
To wherever you point the save sheet: Files, iCloud Drive, or straight into a share. Safari will ask before downloading and the file lands in Downloads unless you choose otherwise.
06How do I sign a PDF on an iPhone?
Open it in Files or Mail, tap the Markup pen, then the plus button, then Signature. You can draw one and save it to reuse. No app needed and nothing leaves the phone.