Kristen,
Dory is correct. While it is possible to host a password-protected .pdf on Marketo and then send out a link to your recipients, that security may not be good enough for your needs. In this case, you should host the file on a secure file storage service (ideally your own platform) and then send an email to your recipients containing a link to the file, requiring them to login securely in order to download the file.
Justin