Decide the file order first
The merge tool outputs every page of each file in list order. Give files clear names first, select or drop several PDFs, then use Up and Down to set the order. Thumbnails help you check the first and last pages and spot duplicates.
Create and check
Click Create file and download the merged result. Open it again and check the page count, orientation and the joins between files. The tool handles up to 20 files, 100 MB and 200 pages in total; remove passwords from encrypted files first.
What needs extra checking?
Merging page content does not keep every part of the original files' structure. Bookmarks, fillable forms and attachments are not guaranteed to survive, and original digital signatures cannot stay valid after rewriting. If these matter, keep the originals and use a desktop tool suited to the document.