Open the document
Click on "Share" in the upper right corner
Next to "private" click "change"
Click "anyone with the link" (under "edit access," click box if you want it open for editing by everyone)
Save, then COPY the link
Then, in the forum post, type "tournament brackets" or whatever to signify the link
Highlight the text and click on the link button in the center of the menu bar (where B, I, U etc, are) and PASTE the link there