If you want to specify a document (rather than a program), and open the document with the default program that Windows opens those kinds of documents with, then use winopen(). It should be just like you double clicked on the file's icon.
If you want to pass in a document to a program on the command line, then you can use dos() or system(). For examples, (1) you wanted to open an image in Photoshop rather than MSPaint or Picture and Fax Viewer (or whatever your default image program is), or (2) say you want to open a text file in wordpad, rather than the default notepad that would open the file if you used winopen(). I have an example for system() if you need to do it that way.