Pdf attachments to emails open up BLANK on Recipient Smartphones - WHY and How to Remedy?

Recently when I have sent pdf's as attachments to Outlook emails, my recipients have complained that they open up when they click on them, but they are B LANK. I checked my own iPhone and find the same thing. WHY??? I know you can send by link and so forth, but why would a simple 2 page pdf open up BLANK on the other end?? BTW, Adobe Support could not answer this question, and Google shows no good ideas. This is a silly situation. THANK YOU for any ideas in reply.