Is there a PDF reader with possibility to change font from one font to another (just for easier reading). It don't need to modify the PDF file's original font, just changed while reading.
Something like this:
A Foxit license or Adobe subscription would allow you to OCR (optical character recognition) the PDF and then edit it. The procedure of getting to editable text would be different per program but relatively simple if you searched "Edit text PDF Adobe/Foxit."
Foxit gives a free 14 day trial and after cost around $150 per license. Adobe Reader has a 7 day free trial and cost $13-$15 dollars per month.
Another option would be to try this http://www.onlineocr.net/. Downside is you need to create an account with them to convert multiple page PDFs.
You don't need OCR except when the pdf is an image pdf. If you just want to change the fonts, there is a program from Softmaker called Flexipdf that lets you just change the font. The program unfortunately is only available for Linux and Windows: http://www.softmaker.com/en/.