Merge PDF

Why merge PDFs locally instead of uploading them?

The documents people most often need to merge are exactly the documents they should be most careful with: signed contracts, bank statements, medical records, tax returns, passport scans for a visa application. Every mainstream online PDF tool handles these by uploading them to a server, merging them there, and giving you a download link that stays live for some period of time.

That workflow means a copy of your document exists on infrastructure you do not control, governed by a retention policy you did not read, for a length of time you cannot verify. For most files that is an acceptable trade. For a scanned passport it is not.

This tool takes the other approach. Merging a PDF is structurally a simple operation — you are copying page objects from several documents into one new document and rebuilding the cross-reference table. That work is done here by pdf-lib, a JavaScript library running in your browser. There is no server involved, which is why there is no upload progress bar and no waiting queue.

What is preserved when you merge

Page content is copied at the object level rather than re-rendered, so text stays selectable, vector graphics stay sharp at any zoom, and embedded fonts travel with their pages. Nothing is rasterised and nothing is recompressed, which means the merged file has exactly the visual fidelity of its sources.

Page dimensions are preserved individually. If you merge an A4 report with a US Letter invoice and a landscape spreadsheet export, the result keeps all three page sizes rather than forcing everything onto one canvas. Most PDF readers handle mixed page sizes without complaint.

Some things do not survive a merge, and it is better to know in advance. Form fields lose their interactivity because two source documents may define fields with identical names. Digital signatures are invalidated, which is by design — a signature attests to a specific document, and the merged file is a different document. Document-level bookmarks and outlines are not carried across, and neither is per-document metadata; the merged file gets fresh metadata of its own.

Working with encrypted or password-protected files

A PDF protected with an open password cannot be merged until it is decrypted, because its page objects are unreadable without the key. If you know the password, remove it first and then merge. If you do not know it, no tool can help you — that is the entire point of the encryption.

PDFs with an owner password (also called permissions passwords, which restrict printing or copying while leaving the file readable) can usually be merged without issue, since the content itself is not encrypted.

How large a file can you merge?

There is no artificial cap here, which is unusual — server-based tools almost always impose one because bandwidth and storage cost them money. The real limit is your device's available memory, since the whole document has to be held in RAM while it is assembled.

In practice a modern laptop handles several hundred megabytes comfortably. On a phone with limited free memory, merging a dozen large scanned documents at once may cause the tab to reload. If that happens, merge in two batches and then merge the results — the operation is associative, so you get an identical file either way.

Scanned documents are the usual source of surprise size. A 40-page scan at 600 DPI in colour can easily exceed 100 MB, because each page is a full-resolution photograph. Compressing before merging is usually the better order of operations.