When something like this occurs, it is usually a problem that stems from having embedded fonts within the PDF. To check if this is the case with your file, open the PDF in Adobe Reader, copy the problematic symbols and paste them into Notepad or MS Word. If they appear different in these programs than they do in Adobe Reader, then you have a case of embedded fonts.
Before opening the PDF in Able2Extract Pro again, click on the View menu and select “Options…” On the General tab, check the "Show unrecognized symbols as..." check box. Once you convert the file again, simply use the Find and Replace command to enter the appropriate symbols.
This article refers to Able2Doc, Able2Doc Professional, Able2Extract and Able2Extract Professional.