Merge PDF

Merging is the most common PDF task after conversion, and the one where privacy matters most. The documents being combined are usually the sensitive ones — a signed contract plus its schedule, a set of payslips for a mortgage application, scanned identity documents for a visa.

How the merge works

pdf-lib reads each source document into memory, creates a new empty document, and copies pages across one at a time. Copying a page means copying its content stream plus every resource it references: fonts, images, colour spaces, annotation dictionaries. That is why the output preserves selectable text and embedded media rather than flattening pages into pictures.

The result is written with cross-reference streams and object streams enabled, which is the modern, more compact PDF structure.

Ordering and page ranges

The list order is the document order. Drag handles make reordering explicit, and each row can be trimmed with a range like 2-5 or 1,4,9. Trimming during the merge is almost always better than merging everything and then splitting: it is one operation instead of two, and you never have an intermediate file containing pages you did not want.

Limitations worth knowing

Digital signatures break. A signature is a cryptographic hash over the document's exact bytes. Any modification — including merging — invalidates it. If a signature must be valid, sign after merging, not before.

Encrypted documents need a password. If a source PDF requires a password to open, the library cannot read it. The tool reports this per file so you can unlock it elsewhere and retry.

Very large merges are memory-bound. Because the whole operation is in-tab, merging dozens of 50 MB documents will exhaust memory on a modest device. Split the job into a few smaller merges.

Interactive forms may need flattening. Copied AcroForm fields from different documents can share field names, which some readers resolve unpredictably. If the merged form misbehaves, flatten the sources to static pages first.