Merge PDF files into one

Does merging reduce quality?

No. Each page is copied as drawing instructions, not as a picture, so text stays sharp, selectable and searchable wherever the original had it. Images are copied byte for byte. A 5.78 MB photo document and a 3.17 MB scan merged into one 8.95 MB file. Scanned pages stay scans.

Why did my bookmarks disappear after merging?

This merger copies only what is drawn on each page. Bookmarks, links and form fields live outside the page drawing, so they are left behind. If you rely on them, use Acrobat's Combine Files, which keeps them. People reading the merged file can still jump by page number.

Can I change the order after uploading?

Yes, until you start the merge. Drag files in the list or use the Move up and Move down buttons. Files are joined top to bottom. Once the merge has run, the order is fixed, and a new order means a new merge, which uses another check.

How pages are combined

The merger opens each uploaded file in list order with FPDI, a PHP library that copies a page into another document as a reusable drawing (a form XObject). Each copied page is placed on a new, empty page of exactly the same width and height, and TCPDF writes the finished file. Text stays selectable and vector drawings stay sharp, because the page content is copied rather than rendered to an image.

You can combine 2 to 20 files in one run. Each file is checked against your plan upload limit: 50 MB on Free, 100 MB on Pro and AI. A successful merge uses one daily check, whatever the number of files.

What survives the merge and what does not

Only what is drawn on the page is copied. Everything that lives in the document structure around the pages, rather than in the page drawing itself, is left behind. We merged a three-page file with three bookmarks, a web link on every page and a fillable text field, plus, in a separate run, a file carrying a valid digital signature, and checked both results.

Measured carry-over from the source files

File size after merging

Images are copied byte for byte, not recompressed, so a merge of image-heavy files is roughly the sum of its parts. A 5.78 MB photo document and a 3.17 MB scanned document merged into an 8.95 MB file with all seven pages.

Small text files can come out smaller than their total. Three TCPDF text files of 6.4 KB, 7.2 KB and 6.4 KB merged into one 10.8 KB file of four pages, because the repeated document overhead was written once and the page content is stored compressed.

If you need the combined file smaller, compress it after merging rather than before. Compressing each part first works too, but you then run the compressor once per file.

Error messages and what they mean

• Merge failed: This PDF document is encrypted and cannot be processed with FPDI. One of the files has encryption. This also happened with a test file that opens without a password but restricts printing and copying. Remove the protection with the unlock tool, then merge the unlocked copy.
• Files that store their objects in compressed object streams, a storage method introduced in PDF 1.5, no longer fail. The merger rewrites such a file with a classic cross-reference table first, using qpdf or Ghostscript, and then imports its pages. Until 12 October 2026 they stopped the merge with "This PDF document probably uses a compression technique which is not supported by the free parser shipped with FPDI."
• The upload is refused before merging: either fewer than two files were added, more than 20 were added, or one file is over your plan size limit.
• The daily limit message appears: guests and Free accounts have 100 checks a day, and a failed merge does not use one.

Before you send the merged file

• Page through the result once. Files are joined top to bottom in the numbered list, so a file added in the wrong position shows up as a block of misplaced pages.
• If the original files had a table of contents built from bookmarks, the merged PDF will not have one. Readers can still jump by page number.
• If any source was signed, the signature is gone from the merged copy. Send the signed original alongside it when the signature matters.
• The download is named merged.pdf and only opens in the browser session that created it.

Merging with Acrobat, Preview or qpdf instead

Acrobat's Combine Files keeps what this merger drops: bookmarks, links and form fields come through into the combined file. On a Mac, Preview joins files by dragging pages from one thumbnail sidebar into another. If the parts carry bookmarks you rely on, or a form that still has to be filled, use one of those. Acrobat Reader, Chrome and Word cannot merge PDFs.

The command-line equivalent is qpdf: qpdf --empty --pages part1.pdf part2.pdf -- merged.pdf. It keeps the links and form fields on each page, opens files with permission restrictions when you add --password=, and runs over hundreds of files in a script. Like this tool, it does not carry bookmarks over.

PDFCheck is the practical choice when nothing can be installed, when you are on a phone, or when a one-off job of up to 20 files should take one check and no setup; mixed page sizes come through per page, as the table above shows, and the page content is never re-rendered. If any source file is digitally signed, no merger keeps the signature valid; send the signed original alongside the combined copy.

Before you merge

• How long is the merged file available? The uploaded parts are deleted the moment the merge finishes, whether it worked or failed. The combined file is kept for download until the hourly cleanup removes files more than 24 hours old, and only the browser session that made it can reach the download; after that the link returns "File not found or expired".
• What happens when one of the files is bad? The run stops at that file, its error message is shown, nothing is written and no check is used. The parts you uploaded are already gone from the server, so fix or remove the file and upload the set again.