Merge PDF files without uploading them

The PDFs people merge are rarely drafts. They are a signed contract and its annexes, a run of invoices for an accountant, a passport scan and a proof of address going to a landlord. The merge is the last step before the file is handed to someone who matters, which is a strange moment to route it through a service you have never audited.

A merged PDF is a new document, but it is not a clean one. Each source contributed its pages exactly as they were, and the result carries document properties of its own: title, author, producer, creation date. If one of the source files was produced on a machine whose account name landed in the author field, that name is now part of a document you are about to email. Open the metadata editor on the merged file and look before sending.

Order is the other thing that goes wrong. Set the sequence deliberately rather than relying on how the files were selected, then open the result and check the first page, the last page and the joins between documents. Very large merges run on your own memory, so if a tab becomes unresponsive, merge in two passes rather than dropping in twenty files at once.

You can confirm the file never left. Open your browser's network panel before you pick the files, keep it visible while you merge and download, and watch that no request carries the document. If you would rather not read a network panel, disconnect from the network completely and do the merge offline; the page is already loaded and the work happens on your machine, so it still completes.