What can I do, for I cannot read documents on screen for long times, but my required readings are too heavy?
e-ink

As also suggested in comments, a quality e-ink reader as (for example) the better Kindle models will provide a paper-like experience for your eyes at a limited weight.

PDFs are not the optimal form for that (ebook formats allow re-flowing to have screen-sized full pages at a font size tha't best for your eyes), but it will certainly work; reading hundreds of pages on that is more like reading a book than a screen.