We have noticed that sometimes, on some computer configurations, that the OHA client forms don't have the "Start Filling" button located below the PDF preview when the client clicks the link in the email.
This is generally a result that the client needs to zoom out to find it. We are integrated with Jotform for these documents, and once in a while, the zoom needs to be adjusted to see the "Start Filling" button. Once they find the button, they can readjust their zoom to whatever they would like.
Sorry for the inconvenience, but we hope that helps!