Are the PDF download versions of the books on your website accessible to assistive technology such as screen readers? See http://www.adobe.com/accessibility/ from more information on this.
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Our PDFs are created using Acrobat Pro and are best read in Adobe Reader. Our PDFs have no DRM on them what so ever, and should be able to be used with a screen reader. However, I do not know if they are specifically created using Adobe's accessibility guidelines. I will pass this question on to our book content team so I can get you a more specific answer.
Thanks for the helpful link. In general, as of right now, our PDFs for sale on oreilly.com include the following features, which help with accessibility:
* Fully searchable text
* Fonts that allow characters to be extracted to text
* No security that will interfere with assistive technology
We're continuing to investigate ways to improve our PDFs for accessibility.