When viewing an article in the portal, the content does not fit within the available widget space and because of this, you need to scroll to the right to see the full content.
The articles causing the issue most likely have hardcoded CSS for an element such as a table or video.
This solution only applies to the portal view of articles.
Some element inside the article is hardcoded to an exact value. Because this is an exact value it will not wrap or stretch within the article widget.
There are two options for how to resolve this.
Option 1) Edit the CSS properties for the content of the article to make it more responsive.
Option 2) Edit the CSS of the KB Article widget instance option or corresponding page to make all elements of that type (such as table) fit within the widgets available space.
1) If the page is in the KM portal, Switch your Application to Knowledge Management - Service Portal (since the instance option is in that scope)
2) Navigate to the portal page which has an article on it.
3) Ctrl + Click on the article and select Instance Options.
4) Edit the CSS to make any table fit into the available space.
This change is considered a customization, and providing an exact CSS Solution to fit your use case would fall outside of the scope of Service Now Support.