The same way you save ANY email attachment... which is going to depend on the email program you use... so check with support for your email program (or the help file to learn how to save an attachment)
When you OPEN a PDF attachment, what program are you using... Acrobat or Reader... both have options to save files
Not an email attachment. I go to my bank account and can display statements. Only option I see is to print not save.
If your bank has done "something" to disable saving, you will need to check with your bank
When I open a PDF from a web site in Acrobat, it has an option to save... if you are not using Acrobat or Reader to open the PDF, you need to find out how to do what you want in the program (web browser?) you are using
OR... right click the PDF and see what options are available in the pop-up window