To add a PDF (or another document such as Word or Excel) into a pop-up box you first need to upload your file to the Resources Manager.
This can be done by selecting the third icon from the left hand menu followed by Resources Manager.
Upload your document and once done, select the copy icon to copy the resource URL.
Now add an iFrame module to your page and paste in the URL that you have copied.
When you view your issue and click on the module it will display the document in a pop-up box.
You can link to a document from a More Information module. Simply upload your document to your Resources Manager, copy the link and add a link within your More Information module.