We're working on crbug.com/61248 and we're hung up on an input problem:
when saving the PDF through clicking the Download/Save button on the toolbar, the lack of a user gesture/activation on the plugin itself prevents a file chooser dialog to appear.

For context, open any pdf, for example https://www.justice.gov/ust/eo/bapcpa/uniform_final_rpts/20090901/docs/7-NFR-Acroform.pdf to see the toolbar. The Download button currently just redownloads a copy of the open document. When filling a form, the information filled in is not saved. This is crbug.com/61248. We're trying to fix this by opening a PepperFileChooser and then writing a new PDF with the filled-in data to the chosen location. https://chromium-review.googlesource.com/c/chromium/src/+/1155257/ is a WIP CL that does that.

The reason PepperFileChooserHost::OnShow() fails is this check:

  if (!host()->permissions().HasPermission(
          ppapi::PERMISSION_BYPASS_USER_GESTURE) &&
      !renderer_ppapi_host_->HasUserGesture(pp_instance())) {
    return PP_ERROR_NO_USER_GESTURE;
  }

The toolbar is not inside the PPAPI plugin, even though it sends an event to the plugin through JS  https://cs.chromium.org/chromium/src/chrome/browser/resources/pdf/pdf_viewer.js?l=512&rcl=c8ce6bcf304347e03fd7448966ae028e1d90bcae which is handled by https://cs.chromium.org/chromium/src/pdf/out_of_process_instance.cc?l=654&rcl=c8ce6bcf304347e03fd7448966ae028e1d90bcae

The event path for a [click in the toolbar save button] vs a [click in the plugin area]:
https://paste.googleplex.com/6385041009541120

In the [click in the plugin area] case, the plugin saves a user gesture here: https://cs.chromium.org/chromium/src/pdf/out_of_process_instance.cc?l=654&rcl=c8ce6bcf304347e03fd7448966ae028e1d90bcae

For the [click in the toolbar save button], we need some way to forward the user gesture to the plugin.