Drop three PDFs into a merge tool and out comes one combined file. Looks straightforward. Under the hood, the tool is making some quiet decisions about pages, metadata, and shared resources. Most go unnoticed; a few are worth knowing.
Pages concatenate in order
Merge PDF appends one file's pages after another's. Page 1 of file A, page 2 of file A, page 1 of file B, and so on. You usually choose the order before merging. Pages stay structurally independent — fonts and images from each source are preserved per page. Mixing page sizes is fine; the merged file will have variable page dimensions (most viewers handle this). For consistency, normalise page sizes in the source files first.
Resources deduplicate
If two source files embed the same font, the merge tool can deduplicate — one copy in the merged file, referenced by both sets of pages. Same with shared images. This keeps the merged file from being the sum of source sizes. Not every tool deduplicates aggressively, which is why compressing after merge often shrinks the result further.
Metadata gets lost or chosen
Source files each have their own metadata (author, title, dates). Merging produces one file, so the tool picks: usually the first file's metadata or fresh metadata of its own. Bookmarks, table of contents, and form fields can sometimes carry through, sometimes get dropped. For important multi-source documents (compiled reports, application bundles), check the merged result before sending — TOCs especially need verifying.
FAQ
Can I merge files of different page sizes?
Yes — the result has mixed sizes. Viewers handle this fine. For visual consistency, resize source files to a common size before merging, or accept the mix.
Will merged signatures stay valid?
Digital signatures usually break when merged because the file's hash changes. For signed-document bundles, merge first then sign the combined result, not the other way.
How big can a merged PDF be?
Practical limits are around a few hundred MB before viewers struggle. For very large bundles, split into parts. Most reasonable merges stay well under that.
Merging is mostly straightforward, with a few quiet trade-offs. Use Flint's merge tool and check the output once before sending.