Do Links to PDF Files Expire in Email Automations?

Hopefully this is the correct thread for this question. I inserted my lead magnet as a pdf into an email, which has been sending via my automations for a few weeks now. However I received feedback from a subscriber tonight that the link had expired. The subscriber took 20 days to open his email which contained my lead magnet pdf. Can someone confirm that links to files expire? And if so, how long do they last before expiry?