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▤ DOCUMENT · PDF-MERGER

PDF Merger

Combine multiple PDF files into a single document in your browser.

local processing no upload archive ready
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▤ WORKSPACE
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▤ NOTES & SPECIFICATION

Before You Send That Multi-Part PDF — What Most People Get Wrong

The most common mistake when assembling a merged PDF is trusting file-name order without verifying it. Operating systems sort files differently: Windows Explorer sorts "Page 10" before "Page 2" in alphabetical mode (because "1" comes before "2"), which means a document with ten scanned pages can end up scrambled if you rely on the OS to pick the order. Rename pages with zero-padded numbers (Page_01, Page_02, ... Page_10) before uploading, or use the drag-to-reorder feature in this tool after uploading.

A second overlooked issue is password-protected PDFs. Most mergers — including this one — cannot read encrypted pages. The tool will reject or silently skip a locked PDF. Always unlock PDFs with the owner password before merging. If you only have the user password (open-password), you may need the original creator to remove encryption first.

What Happens to Bookmarks, Annotations, and Form Fields When You Merge?

PDF merging with pdf-lib preserves page content — text, images, vector graphics — with complete fidelity. What it handles with limitations:

  • Bookmarks (outlines): bookmarks from individual files are not automatically merged into a unified outline in the output. The result has no top-level navigation unless you rebuild it manually in Acrobat.
  • Form fields: interactive form fields from multiple PDFs may conflict if they share field names (which is common in government forms duplicated across many pages). The merged form may appear correct visually but malfunction programmatically. For form merging, flatten the forms first (print to PDF) to remove interactivity and preserve visual appearance.
  • Annotations: comments and markup are preserved on their source pages.
  • Digital signatures: merging invalidates any existing digital signatures — a merged PDF containing a previously signed document no longer has a valid signature. This is expected behavior; the signature was over the original byte sequence, which the merge changes.

File Size After Merging

The merged file size is approximately the sum of the individual files. pdf-lib does not recompress images or content — it copies the page streams verbatim. If you need a smaller output, compress each source PDF before merging, or use a PDF optimizer on the merged result.

How to Use This Tool

  1. Drag and drop your PDF files into the upload area, or click to browse. Multiple files can be selected at once.
  2. Verify the order in the file list. Drag rows to reorder.
  3. Click "Merge PDFs."
  4. Download the single merged file.

All processing uses the pdf-lib JavaScript library running entirely in your browser. No file is transmitted to any server at any point.

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▤ QUESTIONS
01 Why is page order wrong after merging? +

The tool merges files in the order they appear in its list, not alphabetical order by filename. If the order looks wrong, reorder the files in the list by dragging before clicking Merge. Avoid relying on OS filename sort for numbered sequences — use zero-padded names (01, 02) to ensure correct alphabetical sort.

02 Do digital signatures in the source PDFs remain valid after merging? +

No. A digital signature covers the byte sequence of a specific PDF. Merging changes that byte sequence, so all existing signatures are invalidated. If compliance requires signed pages, sign the merged document after assembly, not before.

03 What happens to PDF form fields when merging multiple form PDFs? +

Form fields with duplicate names across files may conflict — only one definition survives. To avoid this, flatten your forms (print to PDF or use "Flatten" in Acrobat) before merging. Flattening removes interactivity but preserves all visible content.

04 Can I merge password-protected PDFs? +

Not directly. This tool cannot read encrypted PDF content. Remove encryption first using the document's owner password, then merge. Most PDF readers allow you to save a decrypted copy via File → Print → Save as PDF after entering the password.