Hi Atiimkwabena,
I suppose you are trying to fill the PDF form using Adobe Reader. (Correct me if I'm wrong)
If yes, then that form might not be reader rights enabled. Adobe Reader being a read only software for PDF is only meant for reading and saving the blank copy of PDF. If you want to edit something in the PDF then the creator has to enable user rights(commonly called as Reader Extend rights) so that you can save the filled PDF. That can be done by Adobe Acrobat.
I would request you to kindly contact the author of the PDF to enable the user rights and send it again.
Hope this helps.
Regards,
~Pranav