Hi Joe - PDFs are a somewhat unusual file format because they can contain either vector data (like structured lines and curves) or also bitmap image data inside of them.

For PDFs that contain vector data, you can bring that in by the regular File > Open or File > Import commands. Those commands involve reading in geometry from different file formats.

Background images are brought in by the View > Image command, but PDF is not supported there as a background image since constructing an image from all the different instructions for vector drawings, text, and bitmaps, etc... in a PDF file is a rather tricky thing to do.

In order to bring your PDF in as a background image for tracing over you will instead need to convert it to PNG format, you can do that by opening it in Adobe Reader and using the Edit > Take a snapshot command in there. That will put the area you select as an image file in the clipboard and you can use an image processing program to paste it in and save it as a regular bitmap image file format. Or also if your PDF has just one big image in it you can try right-clicking on the image in Adobe Reader and use the copy command from there.

Sometime in the future I would probably like to make PDFs as a supported type of background image file, but it's very different to handle them from regular image types since the PDF format contains a big list of instructions on how to draw things, so those instructions have to all be processed and executed properly in order to generate an image.

- Michael