Most modern web browsers have built-in plug-ins that allow users to read online PDF textbooks simply by clicking a link. Older browsers without built-in PDF readers require a plug-in such as Adobe Reader. Some online resources, such as Google Books and Scribd have their own models for using PDF textbooks.
Most web browser PDF viewers allow the user to read the textbook and click any links embedded in the file itself. They also typically allow the user to zoom in on a particular page and to print or download the PDF file. A user may only print or download files that are not protected by a copyright. Web browser PDF viewers usually do not have features such as plain text conversion or text-to-speech functionality.
Google Books and Scribd have more features than typical browser-based plug-ins. Both of these services allow the user to search for keywords in the PDF textbook, and some titles feature plain text and text-to-speech conversion. Not all textbooks are available to view for free on these sites. On Google Docs, proprietary titles generally have a short preview on the site, and Google provides links to buy either a PDF or physical copy of the book. Unlike Google Books, Scribd works by providing users unlimited reading access in exchange for a subscription fee.