File you have opened complies with PDF/A standard and is opened read-only

I have a PDF file I downloaded from the web.  I get the banner message abbreviated above (the full message is "The file you have opened complies with the PDF/A standard and has been opened read-only to prevent modification.").  The file was not bookmarked well and I would like to add further bookmarks for easy reference, but the read-only status prevents that.  How do I turn off the read-only status?  I have checked all the security settings I know of in Adobe Acrobat and see nothing to change there.  There are no read-only settings turned on in the file properties in Windows either.

I am using Adobe Acrobat X (v 10.1.16) on a Windows 7 Home Premium system with Service Pack 1.

Thanks for your help.

Warner