PDR does not distort images unless you actually 'instruct it' to do so i.e. by unticking "maintain aspect ratio".
When you place an image on the timeline it will fill the screen irrespective of the size (in pixels) of the image. If the image aspect ratio is different to that for your project then black bars will appear in the areas of the screen which the image does not fill. That's how it is.
If you are creating a 16:9 project then you should use images of the same ratio i.e. 16 units wide by 9 units high.