Hi Stefan,
Are you looking to bring in an image as a background to use as a reference for modeling?
If so, you can't use a PDF as a background image but you can use the most common image files as a background for reference and then calibrate the image so that it is the correct scale.
To import and calibrate an image to the background:
Select Model > Insert > Attache Canvas

Select the plane you want the image attached to

Click on Select Image in the window and pick the image from your computer that you want to use
Use the manipulator on the image to orient it correctly on the plane don't worry to much about the scale yet.
While still in the Attach Canvas window change the Opacity of the image (I use 45% depending on the image) and click on Display Through so the image won't block you from modeling.

Select OK
Now go to the browser and click on the Canvases folder and the right click on the image file you imported above.
Select Calibrate from the drop down list.

The Calibrate tool allows you to select two points on the image and set the real distance to scale the image to.
On the canvas select two points and enter the desired distance and hit enter. The canvase will scale accordingly.
For my example I chose the lenght of the handle for the BBQ lid and set it to 12 in.


Hope that helps.
Colin
Colin Smith
Sr. Product Manager
SketchBook
Alias Create VR (aka Project Sugarhill)
Automotive & Conceptual Design Group