@ahblake Am I correctly inferring that you plan to keep PDFs on device rather than printing them? Is this maybe to overcome the fact that Collect doesn't show images in the hierarchy summary view visible from the Sent menu? We briefly considered that this could be something someone wanted to do. If I'm making bad assumptions, please do let us know what you have in mind!
I think if we make the filename configurable, it should use the same spec as for files attached to the form (Naming pictures based on values from the filled form). The context isn't quite the same but I think a single spec will be easier to document, maintain, and generally keep straight. Let's consider that a separate feature that we can layer on.
If you try to save multiple of these they'll end up with filenames ODK print.pdf then ODK print (1).pdf etc, right? Would be64432b-a58a-4549-959d-23c95d3fd3bb.pdf, 123e4567-e89b-12d3-a456-426614174000.pdf, etc really be more helpful? If @ahblake thinks it is and we can't think of a downside or good alternative, let's do it.
Or I have been thinking about instanceName as well which could be a way to use meaningful values from the form. The downside is that it feels like it's tying unrelated functionality together. Filenames could get really weird if instanceName uses fields that haven't been populated yet (e.g. First: Last:.pdf). On the other hand, we can fall back to the form name or ODK print if it's entirely blank and saving to PDF is unlikely to be a common use case (as far as we know currently).