I tried to open a document file "C:\\Test Doc.doc" using following code. It worked fine. Please checkout if it would solve your issue.
System.Diagnostics.Process p = new System.Diagnostics.Process();
p.StartInfo.FileName = "C:\\Test Doc.doc";
Note: This code will launch the .doc file in its default application. In my case it is MS Word.
If the system running your code has some other application, say Word Viewer, set as the default application to handle .doc files, you might need a work around to open it in MS Word.