Merge PDF

How to use it

1. Drop your PDFs onto the box above, or click Choose files. Add them in any order.
2. Check the list. Each row shows the file size and page count, and the number on the left is the position it will take in the output.
3. Drag rows to rearrange them, or use the ↑ and ↓ buttons. Remove anything you added by mistake with ✕.
4. Click Merge, then Download merged.pdf.

How it works

A PDF is a collection of numbered page objects plus a catalogue describing the document. Merging does not need to understand or redraw those pages — it needs to copy the page objects into a new document and write a new catalogue listing them in your chosen order. That is what happens here, using the pdf-lib library running in your browser.

Because the pages are copied rather than rendered and re-encoded, nothing is degraded. Selectable text stays selectable, embedded fonts come along, and a vector diagram is still a vector diagram in the output. This is also why merging is fast: the work is proportional to the size of the files, not to how complex the pages look.

Each file is checked as you add it. That check reads the first bytes to confirm the file really is a PDF, then parses the structure to count pages — which is how a damaged or password-protected file gets flagged in the list before you have spent time arranging twelve documents. At merge time each file is re-read one at a time, so only one source document is ever held in memory alongside the growing output.

Tips

• Name your files so they sort in the right order before you add them (01-cover.pdf, 02-report.pdf). The list keeps the order you add them in, so a sensible filename scheme saves all the dragging.
• If the merged file is larger than you want to email, merge first and compress second. Compressing the inputs individually gives a worse result for the same effort.
• Scanned documents are large because each page is a photograph. Merging fifty scanned pages produces a big file however you do it — that is the scan, not the merge.