1. Add every file
Open Merge PDF and drop all the PDFs at once, or add them one by one.
2. Put them in order
Drag the files into the order you want. With a keyboard, move focus to a file and use the reorder controls.
3. Fix pages afterwards
If a page is upside down or out of place, send the result to Organise PDF to rotate, move or delete pages.
Good to know
Bookmarks help readers jump between the parts. Add them afterwards with Bookmarks, and number the pages with Page numbers.
Questions
Can I merge on a Mac or phone?
Yes. It runs in any current browser with nothing to install.
Is there a watermark?
No. The merged file has no watermark.
Related: Why on-device tools are safer · Rotate PDF pages and keep them turned · Remove pages from a PDF