If you get a message when trying to add the Texthelp PDF Reader that it is blocked by administrator there are likely restrictions on adding Chrome extensions on the domain your account is setup with.
Chrome administrators would need to refer to following the instructions in this Chrome for enterprise Help article Manage Chrome apps by organizational unit for allowing the extension.
The Texthelp PDF Reader can be allowed by the extension ID and URL:
Note: The PDF Reader is a private listing that can also be searched by it's product ID: feepmdlmhplaojabeoecaobfmibooaid
For deployment instructions please read How do I deploy the Texthelp PDF Reader