Adobe Acrobat Pro - How to find out what has happened to a file to make it unresponsive to OCR

Hi,

This is my first time posting to this forum, so apologies if I havent followed foru etiquette etc.

I had created a file that included emails and documents that were all searchable.  When I combined these files in Pro the single document was also completely earchable.

I then passed the file to someone else.  When it came back I could not do a search for text, meaning I had to manually search each page.  

I ran the two different files through th compare tools.  I only did a few pages but it came back as if lots of graphics have been added.  Rectangular boxes piled up in the unsearchable file.

It looked as though the font recognized was T168 for example.  When I copied and pasted a few words in the the find tool, it looked like small square boxes.

I am pretty sure that my file has been sabotaged, either intentionally but of course it could have been an accident.  Whatever, something has been done to it and I would like to know what.

I have managed to get it into a searchable format by exporting all of the pages as individual .tiff files.

Has anyone got any ideas of what has been done to the file in the first place?

Thank you for your time reading this message. Hoefully I have explained it well enough to get your feedback and find out what has gone wrong/been done to my file to make it unsearchable.