Text is deleted during redaction

We are redacting large pdf files. During the redaction, the documents will suddenly go blank, completely white, except for where the redactions where on any of the pages.  It doesn't seem to do this with smaller files.  So far we think it is only doing this to files of 50 plus files.  These are files of down loaded and scanned in pages.  It happens on different laptops as well.  We are doing 30 plus pdf files a day of redacting.  Need help figuring this out.  New team starting up and we don't want to look bad to the rest of the department.