How to merge PDF files
Choose the PDF files you want to combine, or drag them onto the drop area. Each file appears in a numbered list showing its size. Use the up and down arrows to change the order and the cross to remove a file you added by mistake. When the order is right, press Merge PDFs. The tool reads each document, copies all of its pages into a new PDF in the order of the list, and gives you a download link for the result.
Private by design
Most online PDF mergers upload your documents to a server, process them there and send the result back. This tool does the same job entirely inside your browser using pdf-lib, an open-source JavaScript library served from this site. Nothing is uploaded, so it is a sensible choice for bank statements, payslips, contracts, medical letters and anything else you would rather not hand to a stranger. You can even disconnect from the internet after the page has loaded and the merge will still work.
What is kept and what is not
Pages are copied exactly, so text stays selectable and searchable, images keep their quality and page sizes are unchanged. A document can mix A4 and Letter pages without problems. Some document-level extras are not carried over, because they belong to the original file rather than to its pages: bookmarks (the outline panel), form field behaviour across files, and digital signatures. A signature would no longer be valid after merging anyway, because the document has changed.
Password-protected files
A PDF that needs a password to open cannot be merged until it is unlocked, and the tool will tell you which file is the problem. Files that only restrict printing or copying usually merge fine. If you own the document, open it in a PDF reader with the password and save or print a copy without protection, then merge that copy.
Tips
To insert only some pages of a long document, extract them first with the split PDF tool, then merge the extracted file with the others. If the merged file is too large to email, try the compress PDF tool. When combining scans, consider converting them with the image to PDF tool so all pages share the same page size and margins.