@mantas84 - if Calibre does not do OCR on PDF's, there's probably a good reason.
I don't know for certain, but I imagine that it might have to do with the paucity of high-quality, zero-cost, open-source OCR engines. The general consensus of people who convert books for a living seems to be that the only OCR programs worth using are commercial ones such as Abbyy Fine Reader.
This thread is worth reading ==>>
Can you OCR the images inside of .pdf files?
BR