Let's take perhaps the simplest most important accessibility feature:
How do I increase the font size without making the content absurdly wide, while viewing a PDF?
As a visually impaired user, this is my single biggest gripe with PDFs. Its also annoying that zooming on many websites completely breaks the layout, often overlapping text or pushing important elements off of the screen. I find this particularly sad, since HTML largely supports a separation of content and presentation.
In the Adobe Reader app for Android on the bottom right you can select multiple 'view modes' one called 'Reader Mode' which will allow you to reflow a document and increase the font size considerably. I capped it out at about a letter per screen.
Let's take perhaps the simplest most important accessibility feature: How do I increase the font size without making the content absurdly wide, while viewing a PDF?