There are 3 possible reasons:
1)The file is damaged
2) The font can't be printed
3) The font is not embedded in the pdf
To avoid the latter, simply do this:
Convert and export your pdf and upload it again:
- Windows user: use a free website such as PDF 24 to convert your pdf
Mac users: press "cmd" and "p" and save your pdf again as a pdf. If you payed for your order your money will be transferred back within 5 working days. If you used your free print it will be automatically activated again.