Cant fill form within Acrobat reader

++adding on to the expert's answers.

You can not fill a pdf form if there's no fillable field or that form is secured and you don't have rights to fill it. 

You can simply right click on the file and check Document Properties>Security>chcek for Filling of form fields and Signing allowed or not. 

Thanks,

Akanchha

Möglicherweise enthält die betreffende PDF gar keine beschreibbaren Formularfelder sondern stellt nur ein Formular dar, dass man per Hand ausfüllen soll.