cant use digital signature obtained from certified authority in adobe reader as it is not showing any sign their
With the limited information, i won't be able to comment much.
There is a possibility the file is secured and you are not allowed to make changes. To check the open file in Reader, File>Properties>Security(Tab), check the Document Restriction Summary.
Also, more importantly, Reader is a freeware and we encourage to use the latest version. You may install Reader DC and check if that helps. Adobe Acrobat Reader DC Install for all versions