You can Preview a Document from your Matter Document list from the Matter Widget, either as a small or large preview.
If you don't already have the File Preview Widget on the right side of your document list, you can select it by clicking the '...' icon at the bottom right corner of your matter screen and ticking File Preview.
Note: Clicking and dragging the 3 lines to the left of the widget name will re-order your widgets.
Small file preview
Select a document in the list with a single click and it will appear in the File Preview widget.
Large file preview
When a document is selected in the list, click on the eye symbol on the top right of the document list.
You can also right-click on a document and select Preview.
To close the large preview, click on the close icon in the top right corner of the view.