Hi @Hamza ,
Try to disable ASM policy first from your Virtual server and see if this fixed this issue or not ,
-if yes you should see what makes ASM to block opening or downloading PDF files requests ,
-If not you should disable http profile from your virtual server and change virtual server type from standard to performance layer 4 " If this available with you and this web site does not serve production services now " , if this issue resolved , I recommend to disable http parsing against PDF files as clarified in the below KB :
but first differentiate if this an ASM issue or not.
I hope this helps you