An addition to Solution 1, once you've written the code such as
using Namespace;
it doesn't include the assembly in your project it just allows you to use the classes inside that namespace in your source file. You have to manually include the assembly to the project. What you need to do is, to right click on the References and select Add Reference. Inside the window that shows up, select the namespace (assembly) that contains the
Document
class. Then this error would go away.