can't open the downloaded pdf on ADE

I bought a book with an acsm file and opened it with ADE. I need the book as a PDF file, to save it and edit it on my PC. So on my mac, when I go to documents > digital editions, there is an ".indb.pdf" but when I try to open it in Preview it says:

The file “XX.indb.pdf” could not be opened.

And I know I can open it via ADE, but I need it as a regular PDF.

I hope anyone can help 🙂