After uploading a new PDF, you now want to link it to some text within your page or post. Every uploaded file is assigned a distinct URL. You need to first get that URL.
There are two ways to obtain the URL to your file.
1) Go through the media library. Click on Media in the Dashboard. Find the PDF and click it. You will need see the Attachment Details open to the right with the URL.
2) Go through the Add Media tab in the Page or Post editor. While editing the Page or Post you wish to link to the PDF, click Add Media above the editor, make sure you're in the Media Library tab and click on the PDF. You'll see the Attachment Details open to the right with the URL.
3) Then either go to the Page or Post or close out of the Add Media tab, highlight the text and click on the insert/edit link button. Insert the URL and either press Apply or Link Options.
Link Options allowed you to set the PDF to open in a new tab, which is recommended for PDFs.