Welcome, Tali3sin.
Take a look at our wiki, which has a page on ebook software:
https://wiki.mobileread.com/wiki/E-book_software
and experiment with the different options.
(You'll also find pages with lists of free ebook sources etc. There's a link to the wiki at the very top of this page.)
I find that it helps if the screen is not too bright, so I dim it a little. Also, I don't want a very wide page, so if I'm reading in Word then I adjust the size of the display by
1. clicking on the view as print layout option
2. adjusting the size of the window by clicking on the restore down button, then pointing the mouse to the bottom right corner and dragging open the window to a convenient size. I want to be able to read a whole line without moving my eyes, so that they don't tire so quickly.