Hello

I have a scanned pdf image from a book. I need to keep it as an image, not as text.
My question is : The original pdf has several lines where fonts are almost unreadable cos of the brightness or bad scan resolution or source failure. Is there any way or software capable of improving the font definition, thats to say to make it look darker or at least more accurate in order to be able to be read?

THanks for your help and excuse my english, i am from South America