First off let me say there are a number of ways to do this.  This is only one way.  There might be better ways for you to add a PDF file to your blog post or page depending on your needs and applications.  This method does not require any additional plugins.

How to add a PDF file to WordPress using the upload/insert image function:

1. When writing your post or page look up above the tool bar in your edit window.  See where it says “Upload/Insert” followed by four icons?  When you hover over the first icon the pop-up will say “Add an Image”.  Click on that.
2. Browse for and upload your PDF file.
3. Once that is done you must fill out the “title” field.  This will be the text that appears in your blog as the link to the PDF file.  You can edit this in the post/page edit window.
4. Put some data in “caption” and “description” to help out your SEO.  This will not be visible in your post/page.
5. Click on “insert in post” or the PDF will not be inserted.

That wasn’t so hard was it?

What if you need to edit this link after you have inserted it?  If you click on the “HTML” tab at the top right of your edit window you can edit the code directly.