Whether PDF is an appropriate format is a different story.
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.
The only solution is some form of legalized archiving. We need the right to copy for archiving without profit, or something. Not sketching out a full, legal solution here, just pointing out that it has to include some form of right-to-archive.
Just throwing it out there - this is (in some sense) one of the intended purposes of the Library of Congress.
The Library of Congress already even announced a few years back that they were planning on archiving every public tweet.
Having them archive every video referenced in official documents (such as a Supreme Court decision) isn't that much of a stretch.
It's just not possible. There's only one way to fix these problems, and it's not something most people are ready to accept. It's to turn the internet inside out, from a client-server model to a store-and-forward model. It's to treat web content more like ebooks than like fliers posted to a cork board.
There are upsides, like decentralization, and downsides, like everybody knowing exactly what data you're asking for if it's strictly addressed by hash (no way of obscuring your political activities from an oppressive regime by using an unrelated name for common data).