There are two possibilities : 1 ) You are editing the PDF with in the browser only. 2 ) You are a MAC user & use default preview app to open PDF. Both, browser & preview app do not support fillable pdfs.
Correct way : First download the assignment PDF in your pc. Open it with a PDF Software (we recommend Adobe Reader. You can get it free here ). Fill in your answers & save the pdf & then upload to the portal.