Hi Petars,
I convert my PDFs using pdflrf (
https://www.mobileread.com/forums/showthread.php?t=13135). It preserves the format of the pages and makes the text a nice size and entirely readable. I recommend trying 'portrait mode' first and if the text is still too small use 'landscape mode'.
The only downside is that pictures that don't fit on one page will be chopped in half, but that doesn't happen too often to me.