Yes, ADE is free - that is the transfer software to get your books onto your reader.
As said, it is free.
As well as transfer, it can also be used for standalone reading of ebooks on the PC.
There are several PC readers for non-DRM ebooks.
ADE can read Adobe DRM ebooks, which most can't. (Sony Reader can).
.. but ADE won't transfer library books to PRS-350.
You should not make generalizations based on your experiences with
DRM-protected library ebooks that don't allow copying and then say ADE
won't copy ebooks to a PRS-350. That's not true. I have a PRS-350 and I
am able to transfer ebooks to it via ADE. The difference is DRM
protection, which you've overlooked in your previous postings. THAT is
preventing your copying an ebook via ADE to any ereader.