The Document Grid allows you to specify formats for columns that contain numbers or dates, or other formattable data types.
To set the format of a column, right-click on it and choose "Number Format...".
The "Choose Format" dialog allows you to specify a custom format string.
Microsoft provides documentation for custom number format strings and date format strings.
If you want to save your formatting choices with your custom report, you can use the "Remember Current Layout..." menu item, which is on the Group/Total button next to the Reports dropdown on the toolbar at the top of the Document Grid.
When a report is exported for external tools, the format used for all numeric columns is always the round-trip format ("R"), which provides the full precision possible for the numeric values. This is the format that is used if you choose "Invariant" as the language when exporting a report at "File > Export > Report".