that is not to say downloading what is legitimately available free and legally online is bad per se. the op posed a question on N. Kapustin, but I expanded my opinion to include a broader net.
if a student does not have the economic means to purchase a score that is not legally available for free download, it can almost certainly be borrowed from a library, or purchased used, etc. If a copy must be made, at least do it via ILL or music library borrow and copy what is needed until the funds can be saved,etc.
But yes, if studying old Public domain works, no big I suppose. I also have much more latitude when it comes to rare works. As an archiver , there are times when an original doesn't exist and cannot be purchased or procured by any other means. It's a necessary grey area and I would just roll w/ it on a case by case basis.
But yeah, to Jmenz point, very seldom are peeps seeking music outside Chopin, Bach, Beethoven, Mozart, Haydn, Bach, Schubert. So that falls outside the realm of discussion here.
Online PDFs are not 'unethical' in their own right. All about context. But almost without exception is copyright works that can be obtained legally (in print and paid download). And if one must work from a copy or PDF, for pete's sake (or 'fred's sake' :-) ), at least obtain the works from someone that purchased ie library, and make the copy for personal study, then try to buy it later if possible, but don't freakin' post it online.