[HarryT]

A decent OCR program, such as Abbyy FineReader, is generally the best way to convert a PDF.

When you say "The same files were flawlessly converted before", presumably that wasn't when you were converting from PDF to some other format, was it? PDF is a "terminal format", which is to say you can convert to PDF, but you cannot convert from PDF without a great deal of effort cleaning up the resulting text.