Most probably this is due to a typographic ligature, characters "fi" being combined to a single character "ﬁ", in connection with incorrect encoding/defects in the font embedded into the PDF. There might be some defects in "ff" character sequences, too.
You can check for ligatures by trying to mouse-select single characters in your PDF reader... if you are unable to select just "f" (or "i"), and the selection "jumps" at this point, you have a strong indication there is neither "f" nor "i" present, but the combined character.
If you are the creator of the document, try to turn off ligatures in your word processor, and print/export to PDF again. (If that doesn't work, try to delete and re-type an erroneous "fi"; maybe the word processor will not remove ligatures already present in the document when turning off this feature.)
If you just got a PDF from somewhere, you are out of luck; the PDF has been created in defective way.