Sure, but the particular PDF I emailed you is immutable (by me). It's sitting in my Sent folder and your Inbox folder in our respective email clients, and we can both be sure what it said.
Notion could implement a feature like "permalink to the content as it was at this point in time". Maybe they already have. But for me to be sure that's an immutable record, I at least have to trust Notion.
I don't see where checksums come into it - either I trust Notion to tell me I'm getting the same document we agreed on, or I need to be able to download the document in a readable form and compute the hash on my client. In which case we're back at PDF again.
Plenty of software can edit pdfs. I have used affinity designer in past to fix up issues in PDFs received from designers.
Seems like this can be better addressed by versioning and audit logs or checksums.