First of all, make sure you have Adobe Acrobat and not free Adobe Reader. It is not possible to edit a PDF with free Reader. Go to menu Help (Windows) or Acrobat (Mac) and click on About Adobe Acrobat/Adobe Reader to know which version you have.
If you have Adobe Acrobat, open the PDF you want to change, go to menu File -> Properties, click on Security tab and verify if you have permission to change the document. Maybe the PDF is protected or is a XFA PDF form.