A small group of pdf file recipients reports inability to see text in newsletter but can see all images?

They can open the file but not view the contents properly. Mostly regular text and text within textboxes seem to be impacted. Text within .TIFF images is OK. I used Times New Roman, Calibri, and Cambria fonts. File views fine for 99% of all who have received it - over 100 people. What could be causing such an issue and how can I help correct the problem? Should I do something different to my doent (MS Word) to prevent this?