When file scanning is on, a proposal link shows how long the document was viewed on each page (if the client agrees to it on the page), flags when it was likely forwarded to someone else, and lets the client accept, ask a question or request a meeting after confirming their email with a one-time code. Link previews by bots are not counted as views.
Time on a page is an observation, not proof of reading. An attached PDF in an ordinary email cannot be tracked at all, because opening an attachment happens on the client’s computer. The surest sign is still the client’s reply or acceptance.