Merge PDF files

Key takeaways

• Pages and text are intact: all 5 merges had the right page count, the same page sizes and the same text on every page, checked by pdf.js.
• The output size is about the sum of the inputs, sometimes smaller: 361 KB of two IRS forms became 166 KB because the writer packs objects together; 7.08 MB of two government documents became 6.91 MB.
• Fillable forms lose their form: 222 form fields in the two IRS forms, 0 afterwards. The 222 field boxes are still drawn on the pages, but the document no longer lists them as a form. Fill the forms in and save them flat before merging.
• Speed: 54 ms to 183 ms for 166 KB to 15.3 MB of output.
• The document title in the Info dictionary is not copied: the merged file had no title where two of the sources had one.

Measured: five merges

In our 5 test merges (up to 201 pages from five files, and a 15.5 MB pair with a 230 page scan) pdf.js found every page, every page size and the text of every page exactly as in the sources. Two limits: the form fields of fillable PDFs are not carried over (the IRS forms had 222 before and 0 after), and the document title is dropped.

Measured with the tool's own code in Node on six public domain US government PDFs: IRS Forms 1040 and W-9, Public Law 118-5, Senate Document 105-10, House Document 110-50, and a 230 page scanned NASA technical report (12.5 MB). Each result was opened with pdf.js, which compared the page count, every page size and the text of every page with the source pages.

Every merge passed all three checks. The size change comes from how the file is written: the PDF writer groups objects into compressed object streams, which shrinks files that were saved the older way and changes scanned reports very little.

Measured: what does not survive a merge

None of the six source documents had bookmarks or link annotations, so we could not test those; if yours do, check the result. Fillable fields are the clear loss: the pages still show the boxes, but the merged file has no form to fill.

Measured: loading and saving alone

Writing a PDF again changes its size by a few percent either way (+0.9%, -4.5%, -14.1%, -0.8%, -0.9%, -1.1%), so sizes in the merge table are not a measure of compression.

About the format

A PDF file is a set of numbered objects: pages, fonts, images and the references between them. Operations that copy pages copy the objects they refer to. Wikipedia lists the filters a PDF uses to store data, including FlateDecode (deflate, for text and lossless images) and DCTDecode (JPEG), which is why scanned pages are large and text pages small.

Before you merge

• Put the files in order with Up and Down or remove one with Remove.
• Fill in and flatten forms first: the merged file does not keep the form.
• Password protected PDFs are not supported; the tool says so and does not guess.
• To merge pictures, build the PDF from them with the image tools.

Free tools:Split a PDFExtract pages from a PDFAny picture to PDFDelete pages from a PDF

How it was checked

pdf.js opened every merged file and compared it with the sources: the page count, the width and height of every page, and the text of every page. All 20 operations across merge, split, extract and delete passed. pdf-lib read the form fields and pdf.js read bookmarks and link annotations before and after. In every merged file the page objects equal the pages shown and there is one catalog, so nothing of the sources is left inside beyond the pages themselves.

Questions people ask

Related

Sources and how we checked

1. Wikipedia: PDF (filters) (read 2026-10-10)
2. PDF.js (Mozilla): the reader used to check every PDF (read 2026-10-10)

Reviewed October 10, 2026. The measurements ran the tool's own code and then opened every PDF with pdf.js and pdf-lib to compare pages, sizes, text and picture bytes. The files and results are in data/seo/hub/evidence/2026-10-10/pdf.