Click the Open button to choose a PDF file to view. If the Spelling dock window is visible any spelling mistakes or double-words will be highlighted. If the annotations dock window is visible any annotations will be visible. (So if you don't want to see spelling mistakes just close the Spelling dock window; similarly for annotations. You can always right click the dock splitter to get them back again.)
You can add or remove words from a local dictionary (in the file words.lst in the same directory as the PDF file). You can also change the dictionary language to any language for which you have dictionary support by clicking the Language button and choosing. (ViewPDF will remember your choice between sessions.)
The Controls, Spelling, and Annotation views are in dock widgets—these can be dragged into other dock areas (in which case they will reshape themselves as necessary), or dragged to float free. The Spelling and Annotations views can also be closed; right click a dock area splitter and check their checkbox to open them again.
Note that the Annotation view's Previous and Next buttons navigate to the previous or next page that has at least one annotation.
Although ViewPDF is a GUI program, if run from a console you can optionally specify a PDF file as its first argument, and optionally specify a page number to start at as its second argument.