How to merge PDFs — and keep everything in the right order

The order problem starts with your filenames

Every merge tool concatenates files in the order it is given them, and most pre-sort your selection alphabetically. That is where the classic mistake happens: scan10.pdf sorts before scan2.pdf, because text sorting compares character by character. If you have ever received a merged report with chapter 10 stapled between chapters 1 and 2, this is why.

Two ways to never hit it again:

• Zero-pad numbers when you name files: scan01, scan02 … scan10 sorts correctly everywhere, forever.
• Use a merge tool that lets you drag files into order after adding them, and glance down the list before pressing merge.

Mixed page sizes and orientations are fine — usually

A PDF has no rule that every page must match: A4 pages can sit next to US Letter, portrait next to landscape. Merging keeps each page's own size, which is normally exactly what you want. The place it bites is printing — a printer asked to produce a mixed-size document will scale or crop somewhere. If the merged file is destined for paper, either normalise sizes first or check the printer's “fit to page” behaviour on page one of each source.

Sideways pages are better fixed before merging: rotate the landscape scan the right way up in the source file, then merge. Rotating after the merge means finding those pages again in a longer document.

Collating: the merge mode with no famous name

Here is a situation that stumps people every time. You scanned a stack of double-sided pages on a single-sided scanner: one pass gave you all the fronts (pages 1, 3, 5…), flipping the stack gave you all the backs (2, 4, 6…) — often in reverse. Appending one file after the other gives 1,3,5,2,4,6 — useless.

What you need is a collate (interleave) merge: take one page from file A, one from file B, alternating — A1, B1, A2, B2 — optionally reversing the second file first. Our merge tool has this as a checkbox, and once you know the word for it, you'll spot it in other software too. It turns a manual page-by-page reordering job into one click.

What merging does to bookmarks, links and metadata

Pages carry their content with them, so text, images and filled form fields survive a merge. Document-level things need more care: the merged file has one title, author and creation date, which typically come from the first source or from the tool. If the merged PDF is going somewhere official, spend the extra minute to set its title properly — the title is what appears in a browser tab when the recipient opens it.

A sensible merge routine

1. Collect the source files into one folder; fix any sideways pages now.
2. Add them to the tool and drag into final order — don't trust the alphabet.
3. Front/back scan stacks → use collate, not append.
4. Merge, then open the result and spot-check: first page, the joints between sources, last page.
5. Name the file for its contents, not the process: lease-signed-complete.pdf, not merged (3).pdf.

As with any tool that touches private documents, it is worth knowing where the merge happens. A browser-based merger that processes files locally — ours never uploads anything — means a signed lease or a medical file is never sitting on someone else's server just to get stapled together.

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