The Reload File button allows reloading the current file.
Quick Access Panel: 
Ribbon: File -> Reload file
Menu: File -> Reload file
Hotkey: Ctrl+R
If the file was changed in another program and you want the changes to be reflected in ABViewer, make use of this command. If any changes in the drawing were performed in ABViewer there will appear a pop up window offering to confirm saving the changes.
Go to ABViewer