How to combine PDF files

Getting the order right

Files merge in the order you give them, which is not necessarily the order you selected them in. Merge PDF lets you drag the list before you start, and checking it there takes a second — fixing the order afterwards means splitting and merging again.

Two traps worth naming:

• Alphabetical is not numerical. A folder of report-1.pdf to report-12.pdf sorts as 1, 10, 11, 12, 2, 3 in most file pickers. Either rename with leading zeros or set the order by hand.
• Page 1 of each file is not page 1 of the result. Printed page numbers are ink on the page and keep saying what they said. If the combined document needs its own numbering, add page numbers afterwards.

If you need to interleave rather than append — the fronts and backs of a double-sided scan, say — merge first and then reorder with organize PDF pages, which shows the pages visually.

What does not survive the merge

• Signatures are invalidated. A digital signature certifies an exact sequence of bytes. Merging produces different bytes, so the signature no longer verifies. That is the signature doing its job. If a signed document must stay verifiable, keep it separate and send it alongside.
• Form fields can collide. Two documents that each contain a field called name now have two fields with the same name, and some readers will treat them as one and mirror what you type. If you are merging filled forms rather than blank ones, flatten each first so the answers become part of the page.
• Bookmarks and internal links. Depending on the tools involved, outlines may be dropped and cross-references may point at the wrong page.
• Page sizes stay mixed. Merging an A4 document with a Letter one gives a file containing both, which prints inconsistently. Resize PDF pages brings them to one size first — see page size and orientation.

After merging

Three quick checks catch almost everything that goes wrong:

• The page count. It should be the sum of the inputs. Page count and file info confirms it, and also shows whether the page sizes are now mixed.
• The joins. Look at the first and last page of each original. Missing or duplicated pages show up there.
• The size. A merged file is normally a little smaller than the sum of its parts, because shared resources are stored once. If it is dramatically larger, something was embedded repeatedly — compress usually recovers it.

Merging happens entirely in your browser: the files are read locally and the result is built on your device, with nothing uploaded. How browser PDF tools work explains the mechanism, and how large a PDF a browser can handle covers what that means for very big documents.