Update Any Windows Phone 8/8.1 To Windows 10 Offline Method

i have a question about pdf and adobe and dropbox. when i download a pdf from dropbox it opens automatically as a chrome html document so functions like fill and sign are not there. i've tried the solutions for pdf viewing in chrome to no avail so someone suggested it might be a setting in dropbox. if you're familiar with this problem is there a chance you can assist me?