Probably the PDF has been secured against editing, and/or you have an old version of Reader, so you can't type on it.

From Adobe Acrobat Help:

The Adobe Reader XI and higher versions of Reader include both the Add Text tool and the ability to save form data. Acrobat users can type in non-fillable forms, add comments, and digitally sign PDFs without extending special rights.


Oorspronkelijk geplaatst op Quora: How do I type on a PDF if I only have Acrobat Reader?"