Combine PDF means take several PDF files and write them, in order, into one PDF. That is also what people mean by merge PDF. This page is the same joiner as the merge PDF tool, with the heading that matches the other search. Add the files, press the button, and download one document whose page count is the sum of the inputs. The work runs in the browser. The files are not uploaded, and there is no account to create before the button works.
Put the file that should be read first at the top of the list. Pages inside a file stay in the order they already have. If you selected a whole folder at once and the order looks wrong, add the files again one by one. A passworded PDF stops the combine for that file. Remove the password in the program that can open it, then add the unlocked copy. This site does not collect passwords and does not try to break them.
Combining does not shrink the download. If the inbox limit is the real problem, combine first so you have one file, then open compress PDF and read the note about pages becoming pictures. Photographs that are still JPG or PNG belong on JPG to PDF before they can sit in this list. Keep the original PDFs. This page never deletes them, and it cannot split the combined file back into the pieces you started with.
A short file and a long file combine the same way. There is no extra charge, no watermark added by this site, and no expiry on the download, because the download is a normal PDF. Large photo scans can use a lot of memory. If the tab struggles, combine half the files, then combine that result with the rest. The originals stay where you left them on disk.