Merging a PDF copies every page of every file into one new document in the order you set. The pages themselves are untouched — same text, same fonts, same resolution — so the merged file is roughly the sum of its parts and nothing is re-rendered or re-compressed. Splitting does the same in reverse, building new documents out of the pages you name.
Because it is a copy rather than a render, both operations run entirely in the browser. The document never leaves the device, which is the whole reason to do it this way for anything confidential.
What survives a merge?
Most of what matters, and a few things do not. Tools rarely say which, so it is worth stating.
| Feature | Survives |
|---|---|
| Text and searchability | Yes |
| Fonts and layout | Yes |
| Image quality | Yes — no re-encoding |
| Links within the same file | Yes |
| Links between merged files | No — targets move |
| Bookmarks and outlines | Usually flattened |
| Form fields | Often, with name collisions possible |
The cross-file link case is the one that surprises people. A link that pointed to page 3 of its own document still works; a link that pointed at a separate file now points at nothing, because that file is no longer separate.
Form fields collide when two merged documents both contain a field called "name" — the merged document then has two fields with one identity, and filling one fills both.
How should a page range be written?
Forgivingly, because everyone writes them differently. Commas or spaces both separate groups, and a trailing dash means "to the end" — so 4- is page four onward.
Asking for 1-50 of a twelve-page file gives you the twelve pages that exist rather than an error, which is the right behaviour for a tool people use in a hurry. Out-of-range pages are ignored rather than treated as a mistake.
Three shapes cover almost every split: one file per page, one file per range, or a single file containing just the pages you picked. The third is the one people actually want most often — pulling four pages out of a forty-page contract.
Why does the file size not fall when you split?
Because fonts and shared resources are embedded per document. Split a forty-page report into forty files and each one carries its own copy of the fonts, so the total is considerably larger than the original.
The same effect works in your favour on a merge: two documents using the same fonts share one copy in the result, so a merged file is sometimes slightly smaller than the sum of its inputs.
If the goal is a smaller file rather than fewer pages, splitting is the wrong operation — the PDF size article covers what actually determines it.
Page numbering is the other thing worth checking before splitting. A PDF page index and the number printed on the page frequently disagree — a report with roman-numbered front matter has a page 1 that is the fifth sheet in the file, and range boxes count sheets rather than printed labels.
When is a PDF the right wrapper at all?
When several images have to travel as one document — a set of receipts, a passport photographed front and back, a signed page photographed rather than scanned. A PDF keeps them in order, in one attachment, with a page count that tells the recipient nothing is missing.
JPEGs are embedded byte for byte rather than decoded and re-saved, so a photo goes into the PDF at exactly the quality it arrived at. That matters more than it sounds: a chain of convert-and-save steps degrades an image every time, and this one does not.
For images that will be printed rather than read, the page size article covers picking a real paper size instead of fitting the page to the image.
What should you check before sending?
Three things, in order. That the page count matches what you intended — an off-by-one in a range is easy to make and invisible afterwards. That the pages are the right way up, since a merge preserves each page’s own rotation. And that nothing confidential came along from a source document you only meant part of.
The last one is the real risk of a merge. Pulling four pages out of a contract is a deliberate act; merging the whole contract in because it was easier is how information travels further than intended.
Questions people ask
Is anything uploaded? No. Both operations are performed on the file in your own browser, which is why they work offline and why the document never reaches a server.
Can I reorder pages while merging? Yes — the order of the files, and within them the order of the pages, is what the merge follows. Getting it right before building is faster than rebuilding.
Does splitting a signed PDF break the signature? Yes. A digital signature covers the whole document, so any modification invalidates it, including one that only removes pages.
What happens to a password-protected file? It cannot be read without the password, so the protection has to be removed first by whoever holds it.
Both operations copy pages rather than re-rendering them, which is what makes them lossless and local. Merge PDF joins documents in the order you set, split PDF takes the ranges back out, and JPG to PDF is the wrapper for images that need to arrive as one file.