To link a post to a PDF (Adobe Acrobat file), you start by clicking the media icon in the WordPress post editor. The icon looks like this: 
You will be prompted to choose a file from your computer and upload it.
The slightly tricky part comes next. You will be presented with a dialog box asking you what you want to do with the file. Select “File URL” and then “Insert Into Post”
In this example, you would wind up with a link inserted into the body of the post, where the text of the link is derived from the file name.
You can then edit the link text to say something like “Read the full story” while leaving the link target the same.
To make the link open in a new window, put your cursor over the link and click the hyperlink icon (
). Under “Target” select “Open link in a new window”