A merge PDF tool puts two or more PDF files into one file, in the order you chose. Combine PDF is the same job under the other phrase people type into a search box. The PDF merger on this page is only that joiner. It does not compress, it does not turn a photo into a page, and it does not unlock a password. You add the files, the browser copies every page into a new document, and you download one PDF. The line under the button is the page count of that new document, which should equal the pages of the inputs added together.
The files never leave the computer. There is no upload bar because there is no upload. The page loads pdf-lib, an open-source library kept with this site, only after you press Merge, and that library runs inside the tab. Close the tab and the merged file is gone except for the download you saved. There is no account, no email field, and no link that expires, because the result is an ordinary PDF on your disk rather than a copy parked on this website.
Order is the order of the file list, top to bottom. If the contract should come before the appendix, add the contract first. Selecting several PDFs at once uses the order the file picker returns, which is not always the order you remember clicking. If that order is wrong, remove the selection and add the files one at a time. Pages inside each file keep their own order. Page 2 of a file still follows page 1 of that file. The new document does not reshuffle, rotate, crop, or renumber the pages you already had.
Failures are named instead of being hidden behind a spinner. A PDF that asks for a password cannot be read here. This tool does not request the password and does not try to strip it. Open the file where it was created, save a copy without the password, and merge that copy. A file that is not a PDF, or a PDF that is damaged, is reported by its place in the list. The rest of the batch is not quietly dropped, and you do not get a partial download that looks finished.
Merging does not make the file smaller. The new PDF is about as large as the inputs added together, plus a little structure. If the goal is a smaller attachment, merge first and then use compress PDF, and read the warning on that page: compression here redraws each page as a picture, so text stops being selectable. If you still have photographs, JPG to PDF makes one page per image, and you can merge that PDF with an existing one afterwards. Doing both jobs in one button would hide which step changed the file. The combine PDF page is this same form with the other heading.
Practical limits are the limits of the tab. A few dozen ordinary pages is comfortable. A scan of a whole binder, or a PDF full of huge photographs, can make the browser use a lot of memory, because the whole result is built before the download starts. If the tab becomes slow, merge in two passes: the first half, the second half, then those two results. There is no printed page quota and no cloud queue that emails you when a job is done. If the browser tab is discarded while you switch away, start again from the files on disk. The originals are never deleted by this page.
What you will not get from a merge is an editor. Text inside a page stays as it was. Form fields, comments, and signatures are copied only as far as a straightforward page copy carries them, and a signature that depended on the original file remaining untouched may no longer verify. This page does not add a watermark, fill a form, or split a PDF back apart. Splitting is a different job and is not offered here, so a merge you regret is undone by keeping the originals. For a photo that is not a PDF yet, start at image to PDF or PNG to PDF, then come back if those pages need to sit beside an existing document.