The first thing to remember is that wordpress.COM does not have a built-in PDF interpreter, so it will not show the contents of a PDF in a post or page on your blog.
This is the way I've found easiest to insert a link to a PDF when I want a line of text in the post or page to be the link for to the PDF.
1. start the post or page that you are going to put the link to the pdf into
2. go to "add an image" and upload the PDF
3. click on the "file URL" button, then copy the URL that shows up in the "link URL" field and close that window without inserting it
4. go to the post, highlight the text that you want to use as the link
5. click the "link" icon in the toolbar and paste in the URL of the PDF you got in step 3 and click insert.
If the file has already been uploaded I do the following
1. start the post or page in the editor
2. go to either "add an image", then click on the "media library tab at the top of the insert media window
3. click on "all types" at the top left of that window so it will show all file types instead of just images
4. click on the "show" link for the PDF
5. copy the file URL out of the "link URL field
6. go to the post, highlight the text that you want to use as the link
7. click the "link" icon in the toolbar and paste in the URL of the PDF you got in step 5 and click insert.