I share PDFs to clients and they correctly receive the email and click the button link which displays the shared PDF.
I'm sure that in the past I've received email notifications whenever they have viewed the file. (The status of the PDF then updates in Adobe Acrobat Pro DC desktop app to say 'VIEWED' or 'OPENED'.)
Recently I've had no notifications and no updates and yet the client HAS received and viewed the PDF. Through much testing, I've worked out that the notifications/updates only trigger if the client is signed in to Adobe (with an Adobe ID) which I find limiting.
I like the assurance that a PDF HAS been VIEWED (and therefore RECEIVED) as part of my workflow and suggest that the act of clicking the link button in the recipient's email SHOULD trigger a VIEWED notification regardless of whether the person is signed in to Adobe or not. Afterall, the email that I am entering to share the PDF is the same as the one receiving the email. Clicking the link implies that they are giving permission to view the PDF from me.
WeTransfer triggers a download notification using the same kind of emailed link.