HD video is 1920x1080 at 72 DPI. DPI doesn't matter once inside of an NLE, pixels are pixels, but inside of a graphics program DPI effects image size. Resize that image to fit in your video frame size. I use Pixelmator Pro to resize images as I find it is the best at it around.
Frame size of project says 612x792 - I'm guessing it automatically scaled there after dragging and dropping pdf's..
If the project itself is 612x792, that's a problem. By default an "automatic" project will assume the characteristics of the first item added to the timeline. You can click on the project icon to select it, then in the Inspector click Modify and change resolution to the proper values. You might want to first make a snapshot of the project to save the current state.
If there's any chance the project is interlaced, the image may appear rough unless View>Show in Viewer>Both Fields is selected.
In general I wouldn't use a complex document type like PDF in an FCP timeline. I'd make a screen cap and use a JPG or PNG. There is less to go wrong that way. A PDF can contain vector fonts, images, etc. There are many internal versions of PDFs. There is no guarantee FCP will handle that perfectly.