All the contents except Youtube based contents can be downloaded.
If you are not able to see the "Download" button for a content , most likely it would be a Youtube content.
Follow the steps to download a content on the DIKSHA APP.
Open DIKSHA APP
Click on the content that you need to download
This will take you to a screen as below
Once the content is downloaded, this can can be accessed by click on the "Downloads" tab.
These contents can also be viewed offline.