Some of your resources may be provided as (Portable Document Format) PDFs. This is so, regardless of whether you are using a PC, Laptop, Smart Phone or Tablet, you will be able to read and annotate the resource.
Some PDF files are created as "forms". This means they have fields you need to fill out. For example, workbooks may be supplied as a PDF Form.
NOTE: When you open a PDF form you are shown a warning that tells you there are fields that need to be filled in (as shown below).