Well there is an open-source application called "pdftk" ( http://www.accesspdf.com/pdftk/ ) which can do several very nice things with pdf files, including saving an unprotected copy, given you have the right password. There are binaries available for window$, macOS, and various linux distros, but not for maemo, unfortunately. I tried to compile this myself, but got lost in dependencies, so if someone smarter is out there, hey! That would be a great addition to IT software.
So, you can unprotect your ebook with pdftk and upload the unprotected version on your tablet.
Concerning the impossibility to download another ebook from ebooks.com, I tried to find some free pdf file to download from there, but with no success, so I cannot help you. But, the book preview from that site works great on my n800!