How to merge PDFs offline in your browser

Merging PDFs is the kind of small job that sends people to an upload site every time: a contract and its appendix, a set of scanned receipts, chapters exported separately. With Merge PDF the whole operation happens in your browser, with no upload and no account, and it keeps working after you go offline. This guide walks through it and explains what is going on underneath.

Step by step

1. Open Merge PDF and press Choose PDF files. Select all the files you want to combine, or drop them anywhere on the page. The first file opens as the document; the pages of every other file are appended in the order you selected them.
2. The Pages panel shows a thumbnail for every page. Drag thumbnails to reorder them. Hold Shift to select a run of pages and move them together, or Select all to move a whole file's worth.
3. Forgot a file? Press Merge PDF… in the Pages panel to append another one, or select a page and press Insert from PDF… to put the new file right after it. Insert blank page adds a separator, and Delete removes pages you do not need.
4. Press Export and choose PDF with all edits. The merged file downloads with the name of the first document; rename it as you like.

Every step is undoable, so you can experiment with the order.

What happens underneath

The tool uses PDFium, compiled to WebAssembly, running in a background worker inside your browser. When you add a file, PDFium opens it as a second document and imports its pages into the first one with the same API a desktop application would use. Fonts, images and vector content travel with the pages, so nothing is re-rendered or recompressed; the merged output has the same quality as the inputs.

Because the work is done in memory on your device, there is no upload and no server copy. The tool's network traffic is limited to its own code and fonts. A working copy of the document is written to the browser's private storage so an interrupted session can be recovered; you can delete it in Settings.

When you export, the document is saved as a full rewrite rather than an incremental update. That matters for merged files: it means deleted pages are really gone and the output does not carry the leftover structure of the source files.

Working offline

The first time you open a tool, the browser downloads and caches it, along with the fonts. After that, the site opens without a connection, and merging works exactly the same way. If you use it often, install the site as a web app from your browser's menu; it then behaves like a desktop application with its own window.

Limits to know about

• Password-protected sources. Files that you add to a merge are opened without a password. If one of them is protected, open it first with Edit PDF, enter the password and export an unprotected copy, then merge that.
• Bookmarks. The outline (bookmark tree) of the added files is not carried over into the merged document.
• Size. Files up to 1 GB are accepted, and the combined document has to fit in your device's memory. For very large merges, a laptop or desktop is a better choice than a phone.
• Form fields with the same name. If two source files contain fields with identical names, a PDF reader will treat them as the same field. Rename fields with Create forms if that is a problem.

Ordering tricks

• Interleaving scans. If a two-sided document was scanned as all front pages then all back pages, merge both files, then drag the back pages into place. Shift-click selections make this quick.
• Cover page first. Select the first file when choosing files so its pages lead; or add the cover afterwards with Insert from PDF… while page 1 is selected.
• Page orientation. Landscape pages from another file are kept as they are. Use Rotate PDF on the selected pages if they should match.

After merging

• Split the result back into parts or extract a range: Split PDF.
• Make scanned pages searchable: OCR PDF.
• Sign the combined document: Sign PDF.
• Remove personal data before sharing: Redact PDF.