You are citing a source. MLA and other schemes require you to give an access date, and the only way to definitively prove you saw content on a given date is to provide the content you saw. Web servers don't magically give users a way to go back in time.
Whether PDF is an appropriate format is a different story.
> the only way to definitively prove you saw content on a given date is to provide the content you saw.
Providing a copy of the content that you claim is the source does not come anywhere close to definitively proving that that content is the content at the cited source on the identified date.
All it does is provide what you claim to be the original source, which, assuming that you do it honestly, provides the content backing the characterizations for which you cited, and the context for any limited excerpts you quoted. So, its useful, but not as proof that you saw the content in the cited source on that date.
this was submitted to HN some time ago. It automatically pulls the website and creates a certificate of content and date that is cryptographically signed.
Whether PDF is an appropriate format is a different story.