In Mail on iPadOS 17.*, that PDF attachment can be tapped and it will open a view of the PDF, but I am unsure if that is a real PDF Reader at that point that would allow you to fill out forms, or just the iPad version of Quick Look found on macOS. I had previously installed the free Adobe Reader on my M4 iPad Pro and that tapped view offers to open the PDF in Acrobat Reader. As PDF forms can vary in their support by an Apple PDF reader, Acrobat Reader might be the right tool for the job.
You will also see a blue icon associated with that PDF attachment that when pressed, offers the option of saving the PDF in the local iPad Downloads, or to Save the Attachment. You might want to do this and then open it with Acrobat Reader, fill in the forms, and then return the completed PDF to the original sender.
Microsoft Word will convert a PDF to a Word document but will not open the PDF for viewing.