When using PDF documents you must ensure that you save the file to your computer before completing it.
If you open the file in your internet browser, this is only a document preview and entering information into the preview will not save the information.
Follow these steps to save the file to your computer here.
Open the file in your PDF editing software such as Acrobat Adobe DC to complete the assignment/assessment task and save your work as your go.
Once complete you will then be able to upload the assessment task/assignment for grading. Instructions can be found here.