I am building an application in which 3D PDF documents are embedded within a web page.
I notice that some 3D PDFs require the user to "Click to activate".
Or is this a property of the PDF file itself?
I have found that I can work around this issue for existing PDFs by "Initializing upon activation":
I have also found an extremely convoluted route in acrobat x pro whereby you can set the activate property to be on opening the PDF.
Nothing "convoluted" about it - the activation choice is a simple setting within the Rich Media annotation.
In Acrobat Pro right-click your 3D scene, choose Properties, then change the activation field to "When page is opened" or "When page is visible" and save the file. Done.