Changing filetype permissions default for a given file type

I work with alot of pdfs, and I always manually change the default permissions to not allow execution of the file and make them read only. I understand how to change permissions on a folder basis after looking at the documentation. I don’t yet understand though how to do it on a file type basis in my home folder. I am only going to change the permissions for pdfs in my home folder. Thank you for your time!

chmod a-wx file.pdf

But why would a pdf file be executable?

The aren’t executable files, it is just the permission defaulted to them. I just wanted that permission not to be default yes. Thank you for your help!