This happens when the PDF is an email attachment. Also, when I open the downloaded PDF it goes directly to print instead of opening. I only want to read it not print it.
Android doesn't have a built in PDF viewer like a PC browser, so the only way to view a PDF file is by downloading it. There may be email apps that have their own built in viewers to work around this, but I'm not aware of any.
Welcome to Android Central! If you don't want the PDF automatically downloaded, check the email settings for an option to automatically download attachments, and turn that off.