Merging PDFs is the most requested document task after conversion - assembling an application pack, stitching a scanned contract back together, or turning a folder of invoices into one file for accounts. The mechanics take seconds. The things worth knowing are what happens to encrypted files and mixed page sizes.
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Merge PDFOrder is decided before you merge, not after
The order of the file list is the order of the finished document, and there is no undo once the pages are combined. Drag the rows into place first and read the list back against what you expect the final document to look like.
A useful habit for application packs: name your files with a leading number - 01-form.pdf, 02-id.pdf, 03-proof.pdf - so they sort correctly no matter what tool you use.
Password protected files have to be unlocked first
An encrypted PDF cannot be read by any tool until the password is removed, and that includes merging. If one of your files is protected you will get a clear message naming it rather than a silently broken result.
Remove the password with the Unlock PDF tool first, merge, and then re-protect the combined document if it needs to stay confidential. That order matters - protecting first and merging second does not work.
Unlocking needs the correct password. No tool can open a PDF whose password is genuinely unknown, and any service claiming otherwise is worth being suspicious of.
Mixed page sizes are the usual surprise
Merging does not resize anything. If you combine an A4 report with a Letter-size scan, the result genuinely contains both sizes, and printers often handle that badly - unexpected scaling, wrong margins, occasionally a jammed tray.
If the merged file is going to be printed or bound, run it through a page size converter afterwards so every page matches. If it is only ever going to be read on screen, mixed sizes are harmless.
When you want split rather than merge
If you are merging in order to extract a few pages afterwards, skip a step: extract the pages you want from each source file first, then merge only those. It is faster and the intermediate file never exists.
Frequently asked questions
Is there a limit on how many PDFs I can merge?
Up to 100 files in one request, 100MB per file and 300MB in total.
Does merging change the quality of the pages?
No. Pages are copied across intact - text stays as text, images keep their original resolution. Nothing is re-compressed.
Can I merge a PDF with a Word document?
Not directly. Export the Word file to PDF first - every version of Word can do this from Save As - then merge the two PDFs.
Tools mentioned in this guide
Split PDF by custom page range (1-5, 8, 11) or every N pages.
Remove owner passwords and printing restrictions from your PDF files.
Check mixed page dimensions inside PDF and convert all pages to one uniform size.
Last updated 4 February 2026. Back to all guides