How do I post a file in a page and then create a link to the page?

First upload the file to your Documents folder. Then create a new page and then use the web part selection tool and choose File viewer. Choose the file you want to embed and click Republish in the top right.  Next, you'll need to choose text to become a link or create new text for the link. Highlight the text and choose link icon from edit menu and select the name of page you want to link to and click republish in the top right.

In the future, if you need to update the file, just replace the file embedded in the page - the page's link doesn't change.