Merge PDF

Merging is the most common PDF operation there is, because documents arrive in pieces. A signed contract comes back as three separately scanned sections. An expense claim is fourteen photographed receipts. A tender submission has to go in as one file when you built it as six. Combining them into a single ordered PDF makes the result easier to email, print, file and read.

This tool copies pages directly from your originals into a new document. Nothing is re-rendered or re-compressed along the way, so the merged file is byte-for-byte as good as its sources — a merged contract reads exactly as the separate signed parts did, at the same quality.

Almost every merge that goes wrong goes wrong on ordering, and the culprit is usually the filename. Selecting a batch of files often lands them in alphabetical order, which puts page 10 immediately after page 1 and buries page 2 near the end. A stack of scans named scan1 through scan12 will assemble in the sequence 1, 10, 11, 12, 2, 3 unless you intervene.

The fix is to check the list before merging rather than the output afterwards. Rearrange the files into the sequence you want, and if you frequently merge numbered scans, name them with leading zeros — scan01, scan02 — so alphabetical order and numerical order agree.

It is also worth confirming that each source file starts and ends where you think it does. Scanning software sometimes appends a blank trailing page, and those blanks accumulate invisibly in the middle of a merged document.

Page content survives completely: text, fonts, images, vector graphics and layout all carry across untouched. That is the guarantee that matters for most uses.

Some document-level features are more complicated. Bookmarks and internal links point at specific locations, and when two documents are combined those targets have to be remapped — most merges handle this, but a link that jumped to page 4 of the second document is worth spot-checking. Form fields carry over, but two forms that use the same field names can collide, so filling one field appears to fill another. If you are merging filled forms, flattening them first turns the values into ordinary page content and removes the problem entirely.

Digital signatures are the significant exception. A cryptographic signature certifies a specific document, and merging produces a different document, so existing signatures will show as invalid afterwards. This is the signature working correctly rather than a fault. If a bundle needs to be signed, merge first and sign the finished file.

Because the merge runs on your own machine rather than a server, it is bounded by the memory your browser can use. In practice this is generous — combining dozens of ordinary documents is unremarkable — but assembling several hundred megabytes of high-resolution scans in one pass can exhaust a tab, particularly on a phone.

If you hit that limit, merge in stages: combine the first group, then the second, then merge the two results. The output is identical and each step stays well within memory.

The trade for that constraint is that your documents never leave your device. Nothing is uploaded, queued or stored, which is what makes the tool appropriate for confidential material — medical records, financial statements, unreleased contracts — where sending a copy to a third-party server would be the problem rather than the solution.