ClearScan over a searchable text? (OCR accuracy)

Hello,

I've got some PDF files with searchable text but unfortunately the readibility is difficult (the letters are not smooth at all).
If  I use the ClearScan option over a document that is already searchable, would it affect the OCR accuracy?

Any help would be greatly appreciated,
Cheers

PS: I was told the correct DPI for ClearScan was 300 and not 600. Right or wrong?