PDF pages are blank when emailed

I've had Gmail completely unattach pdfs from emails.

I started posting a link to a Dropbox folder instead. Or using Hightail or WeTransfer.

As comments have already indicated, PDF is not a single, well-implemented standard across all software makers and apps. The only reliable path is to export using the Adobe export function of Adobe apps, or to print to PDF using the Adobe PDF driver. Using any other tool or feature to create a PDF, including all forms of "PDFwriter" and functions built into apps like Word, even though some bear the Adobe name, is taking some degree of chance, especially if the content involves anything but fairly basic text and graphics.

At the other end, the only truly reliable PDF viewer is Acrobat (Reader, DC, Pro etc.) Most of the aftermarket PDF viewer/editors are limited in their abilities (usually in the name of "smaller," "faster" or just "not Adobe.") The ones built into browsers and apps are especially bad in this respect, being designed for fast, direct viewing of web material and not much else.

So: export your material using print to PDF, not the Word export or "create PDF" feature. Check them before you send them. And if the recipient still has problems with them, politely ask if they're using Acrobat and not, say, Chrome or Foxit. (The extreme difficulty of even asking recipients what they're using — they often don't know, at least, don't know any of the above so "PDF is PDF" to them, and then asking them to use something else... well, that's life with a standard that has no real enforcing agency.)

This doesn't sound like anything to do with InDesign. If the PDF looks okay in Acrobat you're going to have to tell us a lot more about this. Did you actually check the PDF before mailing it?

What PDF viewer is the recepient using? Are you aware that only Acrobat can show all PDFs correctly?