The file you have opened complies with the pdf/a standard and has been opened read-only. If you open a PDF/A standard PDF, in default you will not able to modify it because Adobe PDF reader or Acrobat will open it in view only mode. 
Disable Adobe Reader build in PDF Viewer in Firefox Chrome IE
Firefox, Chrome, and the latest version Internet Explorer now have a built in PDF reader. The built-in PDF viewer is enabled by default. It allow users to view PDF without installing Adobe PDF reader. However, the built-in viewers are not compatible with all PDF files. Some PDF files have special arrangements or contain extra functions would have issues when viewing it with built-in PDF viewer. This viewer might not display all PDF documents correctly. To change the default PDF view in web browsers, follow the instructions to active the Adobe PDF reader plug-ins or add-ons. Google Chrome Open Chrome, Type chrome://plugins in the address bar to open the Plug-ins page. Locate the Adobe PDF plug-in, and do one of the following: