fix: use proper PDF save approach

[codebytere]

Description of Change

Refactors our existing PDF save approach to use the File System Access API and therefore match existing (deprecated) behavior in upstream. I've also upstreamed this at CL:5934369.

Checklist

• [x] PR description included and stakeholders cc'd
• [x] npm test passes
• [x] PR release notes describe the change in a way relevant to app developers, and are capitalized, punctuated, and past tense.

Release Notes

Notes: none