An application that demands a single file, a rental file made of a contract, three payslips and an ID, a course scanned in several pieces: merging PDFs is one of the most common tasks, and the one where getting the order wrong is easiest. Here is how to do it cleanly, without damaging pages and without sending personal documents to a server.
What a merge really does
Merging recompresses nothing: the pages of each file are copied as they are into a new document. Text, images, fonts and layout stay identical. The practical consequence for weight: the resulting file weighs roughly the sum of the originals, sometimes a little less when shared fonts are stored only once. Three PDFs of 2, 3 and 5 MB give a file of about 10 MB.
It also means a merge cannot be used to slim anything down: if the total is too heavy for an attachment, you will then need compression (see How to compress a PDF without ruining it).
Merging step by step with Merge PDF
- Drop at least two PDFs into the tool (200 MB maximum per file). You can also drag them from your file explorer.
- Put the rows in the order you want by dragging them, or with the arrows. On a phone, the arrows are more reliable than drag and drop.
- Check the names and page count of each row: this is the moment to spot a duplicated or forgotten file.
- Run the merge and download the result.
- Open the final file and scroll through it: check the first page of each original document.
A password-protected PDF must first be unlocked with Unlock PDF, if you are entitled to.
Practical tips
- Number your files before dropping them (01-contract, 02-payslip…). Alphabetical sorting puts “10” before “2”, which catches many people out.
- Mixed page formats. An A4 portrait PDF and a landscape or US Letter one are not resized: they keep their size in the final document. That is normal, but awkward when printing; harmonise beforehand if needed.
- File too heavy to send? Merge first, compress afterwards: you measure the saving on the final file.
- Keep the originals. Merging creates a new file and never alters yours, but keep a backup before sending anything.
Common mistakes
- Forgetting to check the final order: a payslip sitting between the contract and the ID makes a poor impression.
- Merging confidential documents on a site that uploads them to a server. With a tool that works in the browser, you can check in the “Network” tab that nothing leaves.
- Assuming a merge keeps the original files' bookmarks: they are not carried over (only web links are). Use the “one bookmark per file” option if you want navigation.
- Merging an electronically signed file: the signature is not preserved in the assembled document.
Honest limits
- Interactive forms and internal cross-references of the original files are not always carried into the merged document.
- A damaged PDF may be refused: the tool says so rather than producing a corrupt file.
Frequently asked questions
Does merging reduce the quality of my pages?
No. Pages are copied without recompression: text, images and fonts stay strictly identical to the original.
How many files can I merge?
There is no fixed number: the real limit is your device's memory (200 MB per file for the simple merge). For dozens of large files, prefer a computer.
Do my documents leave my device?
No. Assembly is done by your browser; no file is uploaded or kept.
Can I keep only some pages of each file?
Merging takes each file whole. To keep only certain pages, first remove the others with the Delete PDF Pages tool, or cut the file with Split PDF, then merge the results.