Redaction causing lines, grides, boxes or however you want to describe it on entire presentation

Does anyone else have  lines, grides, boxes or however you want to describe it appear on the entire presentation after redactions one page?  Is there a work around for this? It seems to happen when I try and redact powerpoint presentations that have been saved as a PDF. PLEASE HELP! I have 20+ documents I need to redact and I can't do it professionally with this error occuring.