I also read many articles claiming that it is legal to watch copyrighted material, as long as it is not redistributed. However I'm not sure that's actually true.

As per https://en.wikisource.org/wiki/Polish_Copyright_Law

Article 23, paragraph 1:

> It shall be permitted to use free of charge the work, which has been already disseminated for purposes of private use without the permission of the author.

Article 6, point 3:

> the disseminated work shall mean a work which, with a permission of its author, has been made available to the public by any means whatsoever

So, it is legal to access the work without permission, if author gave prior permission to make it available to the public. However I'm not sure if selling a book in a store or showing a movie in a cinema would automatically mean that such work can be downloaded freely from the Internet - technically the work is available to the public, but I'm sure the author did not give permission for the work to be available on the Internet.