I wonder if they are aware of the issue at all. It seems they take it for
granted that Adobe's offerings are the only way to go.
I think they should be told that there are people out there who cannot access
their grants on other systems.
https://grants.gov/web/grants/applicants/download-application-package.html
What does it mean they are going to "phase out the legacy pdf application
method"?
See the Apply for Grants page and Workspace Overview page. These pages
provide instructions on the general application process and completing an
application on Grants.gov using Workspace. If you plan to download and apply
using the legacy PDF application method, please be aware this will be phased
out in December 2017. Refer to the Grants.gov Notices page for more details.
Before downloading, you need to ensure your Adobe software version is
compatible with Grants.gov.