How to Merge and Split PDF Files Without Uploading Them – and What Gets Lost
Combining and splitting PDFs is easy, but what happens to bookmarks, form fields, links and file size? We tested it with real documents and explain how to do it privately in your browser.
Merging a contract with its appendix, taking one page out of a long report, sending three scanned receipts as a single file: these are everyday PDF jobs. Most online PDF services do them on their servers, which means uploading your documents first. Our tools do the same work in your browser with pdf-lib, an open-source library under the MIT licence. This guide shows how, and what we found when we tested what survives the process.
Why local processing matters
PDFs often contain personal information: scanned ID cards, bank statements, contracts, medical letters, payslips. Uploading a file means that a copy exists on someone else’s server, at least for a while, under their storage rules and their security. If the document contains other people’s data, for example customers’ or employees’, handing it to an outside service may also have data protection consequences for you.
With Merge PDF and Split PDF, the files are read into the memory of your browser tab, processed there and saved back to your device. Nothing is sent to our server. You can check this yourself in your browser’s developer tools: the network tab shows no upload while the tool works.
How to merge PDF files
- Open Merge PDF and drop your files onto the upload area, or select them.
- Put the files in the right order with the up and down arrows; the tool shows the number of pages and the size of each file.
- Click the merge button. The pages of all files are copied, in order, into a new document.
- Download the merged file and check it before you delete the originals.
A PDF is a collection of objects, such as pages, fonts and images, plus an index that ties them together. Merging copies every page and everything the page needs into a new file. Page content is copied, not redrawn, so text stays sharp and images keep their original quality.
What we tested
We created two test documents with the reportlab library: a 10-page report with an embedded font, a photo, bookmarks, an internal link (“Go to the last page”), a form field and a title in its document properties (280 kB), and a 3-page appendix (25 kB). We then merged and split them with exactly the code our tools run, and inspected the results with the pypdf library.
| Feature | Originals | After merging |
|---|---|---|
| Pages | 10 + 3 | 13 |
| Text, fonts and images | Present | Unchanged |
| File size | 280 kB + 25 kB = 306 kB | 296 kB (3% smaller) |
| Bookmarks (outline) | 10 + 3 | None |
| Internal links | Work | Still clickable, but lead nowhere |
| Form field | Fillable | Visible, but no longer registered as a form field |
| Document title | Annual Report | Removed |
The result is clear: everything that belongs to the pages survives perfectly. Everything that belongs to the document as a whole, such as bookmarks, the form structure and the document properties, is not carried over. Internal links still exist on the page, but they point to pages of the original file, so clicking them does nothing.
How to split a PDF
Split PDF has two modes: extract a selection such as “1-3, 8” into a single new file, or save every page as a separate file. Our measurements show why the choice matters:
| Result | File size |
|---|---|
| Page 5 of the report (text only) | 21 kB |
| Page 1 of the report (text and photo) | 269 kB |
| All 10 pages as single files, added up | 461 kB |
| The original report | 280 kB |
The ten single-page files together were 65% larger than the original, because each of them carries its own copy of the embedded font, and the page with the photo carries the whole photo. If you need to send several pages, extract them into one file instead of many.
Why are some PDFs so large?
Almost always because of images. In our test report, the one photo on page 1 took up 246 kB of the file’s 280 kB, almost 90%, while ten pages of text with an embedded font needed the rest. A few habits keep files small:
- Scan text documents at 200–300 dpi; 600 dpi rarely helps legibility but multiplies the size.
- Shrink photos before you turn them into a PDF, for example with the Image Compressor and the Image Resizer.
- Send only the pages that are needed: extracting three pages into one file is often the simplest way to get under an email attachment limit.
Password-protected PDFs
Encrypted PDFs cannot be processed: pdf-lib does not support encrypted documents, so our tools show an error message. This applies not only to files that need a password to open, but also to files that open normally and only restrict printing or editing with a so-called owner password. We tested both types. If you are allowed to, open the file in the program you received it with, save a copy without protection and use that copy.
Photos and scans to PDF
Images to PDF places one image on each page, on A4, US Letter or a page the size of the image, with optional margins. The images are stored as JPEG at quality 90 and limited to 4,000 pixels on the longer side, which keeps scanned text sharp but files manageable. Crop photographed documents first with the Crop Image tool. If you need the text itself rather than a picture of it, our guide to extracting text from images explains how to get accurate results.
Checklist
- Fill in and save forms before merging.
- Keep the original files if you need their bookmarks or document properties.
- Check the page order before you download the merged file.
- Extract several pages into one file rather than many single files.
- Remove a password first, if you are allowed to.
- Give files clear names, for example “2026-10 Invoice 1234.pdf”.
Tools used in this guide
-
Merge PDF
Combine several PDF files into one document in the order you choose. Free, no upload — the PDFs are merged in your browser.
Open tool -
Split PDF
Extract pages from a PDF or split it into single pages. Choose page ranges like 1-3, 5, 8- — free and without uploading the file.
Open tool -
Images to PDF
Combine JPG, PNG or WebP images into a single PDF. Choose A4 or US Letter, orientation and margins – created locally in your browser.
Open tool
Frequently asked questions
Is it safe to merge PDFs online?
With tools that run in the browser, like ours, the files never leave your device. With services that upload your files, a copy is stored on their servers at least temporarily, so read their privacy policy before using them for personal documents.
Why did my bookmarks disappear after merging?
Bookmarks belong to the document, not to individual pages. When pages are copied into a new file, the bookmarks of the original files are not carried over. Keep the originals if you need them.
Why is a single extracted page almost as large as the whole file?
A page takes along every font and image it uses. If the page contains a large photo or an embedded font, that data is copied with it, even though the page is only one of many.
Can I merge password-protected PDFs?
No. Encrypted files cannot be processed, even if they open without a password and only restrict printing or editing. Save an unprotected copy first, if you are allowed to.
Does merging reduce the quality?
No. Pages are copied, not re-rendered, so text and images stay exactly as they were. Only images converted with Images to PDF are newly saved as JPEG.