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.
PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for use in the archiving and long-term preservation of electronic documents. PDF/A differs from PDF by prohibiting features ill-suited to long-term archiving, such as font linking (as opposed to font embedding) and encryption. The ISO requirements for PDF/A file viewers include color management guidelines, support for embedded fonts, and a user interface for reading embedded annotations (From Wikipedia).
PDF/A format is important and mainly for business use. It’s a format that ensures a PDF file can be reproduced exactly the same way because all necessary information for displaying the document is embedded in the file. It is safe, It is safer and securer for long run. Actually Adobe PDF reader or Acrobat have the option to turn off the PDF/A view only mode.
How to open a PDF/A PDF file without read-only mode?
Edit>Preferences>Documents>PDF/A View mode.
Set view documents in PDF/A mode to "never".
You can revert back to the default setting anytime you want to.