I spoke with a few colleagues of mine, and members of our accessibility interest group, and here is what they had to say:
I can say definitely there are no plans for other PDF web viewers. The accessible option here is for the user to download the PDF – of course that depends on the PDF itself being accessible.
On that note, just a few words about PDF itself and accessibility: this format was developed to be portable (the “P” in the acronym) while preserving the graphic integrity (font family, size, color, grid, etc…). It was not developed to be accessible by default though, it was not part of its core. All we do are remediation to make it a little bit more accessible. That said, we rely on the available technology for authoring (Text processors like MS Word, Libre Office, inDesign) and viewing (Acrobat Reader, browser plugin viewers), their features and limitations. We know that PDF is the most used format for galleys, but, unfortunately, it is not the least accessible though.
As noted, the accessibility of a given PDF depends on those who created it. We do offer some guidance on creating accessible content on our guide here, as well: Creating Accessible Content: A Guide for Journal Editors and Authors
I hope this helps shed some light on the issue.