Yes, you can use multiple PDF files.
You can import the main file from the following page: Import a file
Then once in the editing application:
- click on the purple camera-shaped button (bottom right)
- click on "PDF / Word / Powerpoint"
- then select the next file
The file is then imported, and you just need to arrange the pages.
If you need to import new files, repeat the process by clicking on the purple camera-shaped button again.