The Theodore Presser website now sells regular printable PDF's for many publications, including those chamber and recital works that some retailers don't carry as advance stock on hand. That's in addition to having everything in print in hard copy, and the "App Specific" options to download directly into forScore or Newzik apps.
Ellen’s works that are available digitally are indicated as “Digital Available” here: https://www.presser.com/ellen-taaffe-zwilich