I merged 4 PDFs and the bookmarks panel is empty in the output. What happened?

pdf-lib preserves bookmarks from the FIRST source PDF only by default — that's a current API limitation of the library. If you need the full bookmark tree from every source, the workaround is to open the merged PDF in Acrobat or a desktop tool and use "Page → Add Bookmarks from Page Range". We have an open ticket to wire up cross-document bookmark stitching once pdf-lib lands the upstream API for it.

Will it preserve a digital signature I already applied to one of the source PDFs?

Signed PDFs become invalid after any structural change, including a merge — the signature commits to a specific byte range of the original file, and concatenation changes those bytes. This is a PDF spec constraint, not a tool limitation. The right workflow is: merge first, THEN apply signatures (use /pdf/sign-pdf on the merged result). If you absolutely need the original signature intact, do not merge — attach the signed PDF separately.

How big can my files be?

No hard cap from us. On desktop with 8+ GB RAM, merging 30 files of 50 MB each (1.5 GB total) works fine and takes 10–20 seconds. On an iPhone 14, the practical limit is around 30 files at 5 MB each because Safari starts evicting tab memory above ~1 GB of working set. If a merge fails silently on mobile, that's the cause — switch to desktop or split into two smaller merges.

I need a specific page from PDF B inserted between pages 3 and 4 of PDF A. Can the merger do that?

Not directly — the merger concatenates whole files in order. For mid-document insertion, use /pdf/organize-pdf instead, which lets you drag individual pages from multiple PDFs into any sequence. The merge tool here is optimised for the common "concatenate N whole documents" case where most users land.

What about form fields with the same name across multiple PDFs?

pdf-lib will keep both fields and the values they were filled with at merge time, but downstream form behaviour is undefined — most readers treat same-named fields as "linked" and may sync values. If the merged result is going to be re-filled by someone else, rename conflicting fields BEFORE merging (use /pdf/edit-pdf to inspect field names).

Why does the merged file open with "the structure is invalid" warnings in some readers?

Two known causes: (1) one of the source PDFs has a corrupt xref table that pdf-lib silently repaired during the merge — the warning is downstream propagation, the merged PDF itself is fine; (2) the source was a "linearised" PDF (web-optimised) and the merger output isn't — that's expected, just re-save through a PDF tool that linearises if you need it. Acrobat and Preview both ignore both warnings.

Can I merge encrypted PDFs?

No — pdf-lib refuses to read encrypted PDFs (correctly, since the merge would silently expose protected content). Unlock the source files first with /pdf/unlock-pdf (you need the password), merge, then re-encrypt the result with /pdf/protect-pdf if needed.

Merge PDF

Combine PDFs in the order you want with the easiest PDF merger available.

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What is Merge PDF?

PDF merge is one of those tasks that looks trivial until the third tool you try eats your bookmarks, flattens your form fields, or destroys the digital signature on the cover page. This tool uses pdf-lib running inside your browser tab to concatenate the source files at the object level — every page is copied as-is, the cross-reference table is rebuilt cleanly, and the structural elements that other mergers routinely strip (outlines, named destinations, AcroForm fields, page labels) are preserved. Why in-browser? Because the alternative is uploading 5–6 confidential documents to a server you don't own. Most "free PDF merge" sites do exactly that and then run the same pdf-lib (or qpdf) call we run, just with the round-trip and the unknown retention policy added. Your contract, lease, tax return, medical record, or signed agreement never leaves the tab here. Close the tab, the file is gone.

Why use this tool?

pdf-lib copies pages with `copyPages()` — that's the API guarantee that source page content streams aren't touched. So embedded fonts stay embedded, form fields keep their default values, annotations keep their target coordinates, and signatures keep their ByteRange. The merged output validates as PDF 1.7 in `qpdf --check` (we test this) and opens identically in Adobe Acrobat, Apple Preview, Chrome's built-in viewer, Foxit, and pdf.js. No file size cap. No daily quota. No watermark on the output. The number of files you can merge in one operation is bounded only by your device's available RAM — Chrome on a 16 GB laptop handles 100+ files at 5 MB each comfortably; iPhone 14 Safari starts swapping above ~30 files total. If the merged result needs to fit an email cap, run it through compress-pdf after — usually trims 15–30% because the embedded font tables and ICC profiles deduplicate well.

Common use cases

The recurring use cases we see: putting together a complete loan/mortgage package from a stack of bank statements + ID + payslips + employer letter; assembling a visa application from passport scan + photo + DS-160 confirmation + appointment letter; compiling an expense report from a month of restaurant + Uber + flight PDFs; building a complete contract from a base agreement + signed amendments + exhibits. In each case the order matters, the package is sensitive, and re-doing it because a server tool broke the bookmarks is genuinely painful. It's also the right tool when you've received a multi-part document from someone who exported chapters or sections separately ("Section 3 amended.pdf", "Section 3 amended v2.pdf", "Section 4.pdf"). Drop them all in, drag into the right order, get one clean PDF back.

Frequently asked questions

I merged 4 PDFs and the bookmarks panel is empty in the output. What happened?: pdf-lib preserves bookmarks from the FIRST source PDF only by default — that's a current API limitation of the library. If you need the full bookmark tree from every source, the workaround is to open the merged PDF in Acrobat or a desktop tool and use "Page → Add Bookmarks from Page Range". We have an open ticket to wire up cross-document bookmark stitching once pdf-lib lands the upstream API for it.
Will it preserve a digital signature I already applied to one of the source PDFs?: Signed PDFs become invalid after any structural change, including a merge — the signature commits to a specific byte range of the original file, and concatenation changes those bytes. This is a PDF spec constraint, not a tool limitation. The right workflow is: merge first, THEN apply signatures (use /pdf/sign-pdf on the merged result). If you absolutely need the original signature intact, do not merge — attach the signed PDF separately.
How big can my files be?: No hard cap from us. On desktop with 8+ GB RAM, merging 30 files of 50 MB each (1.5 GB total) works fine and takes 10–20 seconds. On an iPhone 14, the practical limit is around 30 files at 5 MB each because Safari starts evicting tab memory above ~1 GB of working set. If a merge fails silently on mobile, that's the cause — switch to desktop or split into two smaller merges.
I need a specific page from PDF B inserted between pages 3 and 4 of PDF A. Can the merger do that?: Not directly — the merger concatenates whole files in order. For mid-document insertion, use /pdf/organize-pdf instead, which lets you drag individual pages from multiple PDFs into any sequence. The merge tool here is optimised for the common "concatenate N whole documents" case where most users land.
What about form fields with the same name across multiple PDFs?: pdf-lib will keep both fields and the values they were filled with at merge time, but downstream form behaviour is undefined — most readers treat same-named fields as "linked" and may sync values. If the merged result is going to be re-filled by someone else, rename conflicting fields BEFORE merging (use /pdf/edit-pdf to inspect field names).
Why does the merged file open with "the structure is invalid" warnings in some readers?: Two known causes: (1) one of the source PDFs has a corrupt xref table that pdf-lib silently repaired during the merge — the warning is downstream propagation, the merged PDF itself is fine; (2) the source was a "linearised" PDF (web-optimised) and the merger output isn't — that's expected, just re-save through a PDF tool that linearises if you need it. Acrobat and Preview both ignore both warnings.
Can I merge encrypted PDFs?: No — pdf-lib refuses to read encrypted PDFs (correctly, since the merge would silently expose protected content). Unlock the source files first with /pdf/unlock-pdf (you need the password), merge, then re-encrypt the result with /pdf/protect-pdf if needed.

Pro tips

1If a single source PDF is huge (50+ MB), compress it FIRST with /pdf/compress-pdf before adding to the merge — it makes the merge faster and the output smaller, because pdf-lib doesn't re-compress already-merged streams.
2For monthly batches (invoices, statements), name your source files with a zero-padded prefix ("01_invoice.pdf", "02_invoice.pdf") so the natural sort order matches the merge order — saves you from manually dragging thumbnails every month.
3Before clicking Merge for a contract package, double-check the LAST page of each source — execution dates and signature blocks live there, and a wrong-order merge usually shows up first as a date that's out of sequence.

How does it compare?

Adobe Acrobat's combine-files feature works well but starts at $14.99/month. Smallpdf and iLovePDF run the same pdf-lib/qpdf code we do but upload your files to their servers and watermark the output on the free tier (or cap to 2 merges/day). PDF24 and Sejda are the closest free competitors — both upload to a server. This tool is the only common option that keeps the file on your device throughout.

Related guides

Editorial walk-throughs that go deeper on the workflow most people use this tool for.

How to Merge PDFs on iPhone (No App Required)

Combine PDFs from Files, iCloud Drive, or email into one document — fully on your phone.

How to Put Both Sides of an ID on One Page

A common merge use case — combining front and back scans into a single printable PDF.

More free tools. No signup, no watermarks.

Split PDF

Separate one page or a whole set for easy conversion into independent PDF files.

Organize PDF

Sort pages of your PDF file as you wish. Delete PDF pages or add PDF pages.

Rotate PDF

Rotate your PDFs the way you need them. You can even rotate multiple PDFs at once.

Compress Image

Compress JPG, PNG, SVG or GIF with the best quality and compression.