How does one insert pdf files in this forum?

There is an image insert "tag" link just above the edit screen (below the smileys) click that and put the URL for the image in between the opening and closing tags.

When you choose the reply button... you can see the tags used to insert the winky face.

PDF files are a unique case. RCU, like many forum boards, does not allow direct posting of PDF files. You can add a URL link to the offsite location but you can't bring the image here directly. Supported image formats are listed when you click on the "Upload Images" function at the bottom of the reply panel.