Should I set document version field to be part of a primary key
The DocumentVersion table makes sense to me.  The two columns well define the Document by the DocumentID relationship to the Primary Key on Document and the Version makes a unique entry for DocumentVersion.  (Just as the name implies.)

There is no problem having a primary key that included two different data types, so an INT and a DECIMAL(5,2) will coexist happily in the same Primary Key.

What you do want to avoid is having a join between mismatched data types, such as joining a VARCHAR(10) with an INT. This will likely result in problems with the join performance even when all the data is clean.  (And, of course, a VARCHAR can hold strings that would not convert to INT which would be another class of problem.)

EDIT: Depending upon your usage, a surrogate PK column DocumentVersionID could be beneficial for other tables to reference.  However, for integrity of tracking the versions, the DocumentID,Version should at least be enforced by a UNIQUE CONSTRAINT.

E.g.  ... CONSTRAINT Unique_DocumentVersion UNIQUE (DocumentID, Version);