He didn't attach opus numbers to all his pieces. I already told you that it's Sz 80. The BB number is 88. But Bartok only wrote one piano sonata, anyway.
[file removed by admin]
In the huge majority of cases, any sort of score-sharing is illegal, because the publisher owns the right to their edition of the publication.
The IMSLP/Petrucci Library was the topic of a recent New York Times article, in which issues of legal challenges and copyright laws were examined:"Free Trove of Music Scores on Web Hits Sensitive Copyright Note"
Only if it's still in copyright! For instance, I could quite legitimately scan and share to my heart's content my Peters Edition of the Beethoven violin sonatas, because the copy (the actual physical copy I have) dates from 1895 and is blatantly out of copyright and Peters has no claim on it whatsoever. However, if I took a new edition of the same music that would almost certainly be in copyright and sharing it would indeed be illegal.
As I've mentioned elsewhere, the problem today is that some people simply expect to get their sheet music (or scores in downloadable/printable form) for free