Beside the legal aspect, do you think it is okay to have a one-click link in an e-mail to accept TOS, knowing that the TOS document is an attached PDF?
If the email clearly states what clicking the link does, it might be okay but there are many other things to consider. Rather than attaching it as a PDF, include it in the email if possible. Rather than have the link auto accept the TOS, have the link bring to a page that allows the user to view and accept the TOS.
TOS should also be accepted before a user finishes setting up an account, as in, when providing the email, make them accept the terms of service their instead of sending a separate email for accepting terms of service. If they have already accepted it, an email simply notifying them of updated TOS is acceptable.