How to be able to fill in a PDF document?

I have received a PDF document which I am supposed to fill in the spaces. However I can’t. I check the Document Properties -> Security. It has the following. I wonder what could be wrong.

Printing: Allowed
Document Assembly: Not Allowed
Content Copying allowed
Content Copying for Accessibility: Allowed
Page Extraction: Allowed
Commenting: Not Allowed
Filling of form fields: Allowed
Signing: Not Allowed
Creation of Template Pages: Not allowed