Adobe Acrobat showing some text as white boxes in downloaded PDFs

I work for a law firm and many users are getting medical records where the characters are showing up as little white boxes and are unreadable. The only work around i have found was exporting the document to a word file. But some people have PDF documents that have thousands of pages and that will take to long to convert. There are no updates and a repair doesnt help. Does any one have any resolution to this issue? This only happens with medical records.