You can monitor PDF from Distill cloud at https://app.distill.io/watchlist. Click on down caret icon to the right of "Add webpage" button and select "PDF". PDF files can not be monitored from browser extesnions.
Browsers do not allow some webpages to load in background. By default, Distill opens them in a new tab and checks for changes. Check out https://distill.io/kb/help/extensions-open-new-tab-for-checks/ to know how to prevent checks in a tab.
You can monitor XML from Distill cloud. Login to your Distill account at https://app.distill.io/watchlist. Click on down caret icon present after "Add Webpage" button and select XML. XML can not be monitored from browser extensions.