When I export a pdf from InDesign, all is fine, preflight is perfectly happy. But then I email it to my clients, and they get an empty pdf. I don"t know how to solve this. It does not happen all the time, or always with the same clients. Sometimes emailing again helps, sometimes not.
Does anybody know why this happens?
Assuming your are viewing it fine in Acrobat or Reader before emailing it, it might be their PDF viewer (like Preview on a Mac, Firefox's PDF viewer, etc.) or it might be the attachment to the email is corrupt. If the former problem, tell them to use Adobe Reader or Acrobat to view the PDFs. If the latter problem, try compressing it (like as a ZIP file) before attaching it to the email. Or upload it to dropbox.com or your own domain and send them a link.
Check the file in your email Sent Items folder is OK - if so it's their problem. Some corporate firewalls eat binary attachments, some third-party PDF software messes up the page display, and some users are clueless. InDesign produces standards-compliant files but it's not your fault if they're trashed by the recipient.