Font change to special character when reducing PDF

When reducing multiple PDFs the font is changing to special characters for some combinations of letters. In this case the combination ff changes to the special character shown in the attached image. There are however other instances of the combination of letters ff which are not affected throughout the PDF. All of these are the same font. Font is Stone Sans II ITC Std Lt.

Does anyone have any idea why it is changing? Any help would be greatly appreciated.

Thank you.