If you are unable to open PDF attachments in Yahoo Mail, first try restarting your computer or device. Other programs might be interfering with Yahoo's ability to access your PDF reader program ...

Can't view PDF files? Unable to open PDF Files? PDF Files will not open in Internet Explorer 11? Learn how to fix the problem & make IE open PDF Files.

InDesign may fail to open a PDF if the PDF file is protected by a password, it is very large, or if it is damaged. Here's the fix.