Merge PDFs

Combining PDFs here means creating an empty document and copying each source file's pages into it in turn, carrying along the fonts, images and vector content each page refers to. No page is rasterised or recompressed on the way through, so the merged file is made of the same page objects the originals were, and text in it stays selectable and searchable.

Order is whatever order the files are in when you start, so arrange them before merging rather than hoping to fix it afterwards. Alphabetical filenames are the usual trap: a scanner that produced scan2 and scan10 will offer them in the wrong order to anything that sorts as text.

Mixed page sizes stay mixed

Page dimensions are a property of each page, not of the document, so a merged file can perfectly well contain A4 pages followed by US Letter pages followed by a landscape spreadsheet. Nothing is scaled or padded to match. That is usually what you want, since rescaling would change the printed size of everything, but it does mean a document assembled from mixed sources can print with inconsistent margins. If uniformity matters more than fidelity, normalise the pages before merging rather than expecting the merge to do it.

What does not come across

Anything attached to a source document as a whole rather than to its pages has no place to go in the new file. Bookmarks and outline trees, interactive form fields and their values, and document-level metadata such as the original title and author belong to the documents being merged, not to the pages being copied, so they do not appear in the result. Page content comes through intact; the navigational and interactive layer around it does not.

Encrypted or password-protected source files cannot be merged either, since their pages cannot be read without the password first being removed by whatever applied it.

Expect the size to add up, roughly

A merged file lands near the sum of its inputs. It can come out slightly smaller when the sources share fonts, since a font embedded in five separate documents may be stored once in the merged one. It will not come out dramatically smaller, because the images and content streams are copied rather than recompressed. If the goal is a smaller file rather than a single one, merging is the wrong tool and compressing the result afterwards is the right one.