Once you've located an accurate top view of your part, you need to get a decent-sized copy of the image into your favorite photo editing program. With most PDFs you can just zoom in on the PDF so the image area in question is full screen, take a screenshot and then paste this image into your image editor, removing everything except the top view (you might need to use a white paint-brush for example):

It's important to have measurements in the image at first since we'll need these to create a properly scaled image. In this particular image, the connector is 14mm across the X axis. Using this number, we'll measure the actual width of the image in pixel across the X axis, which turns out to be 643 pixels from one edge to the other.

The way you do this changes in different editors, but often you can use the rectangular selection tool and see how many pixels wide/tall your selection is.

Since we now know that the bitmap image is 643 pixels wide for a real-life width of 14mm, it's easy to figure out how to scale the image to a useful size. Simply divide the mechanical part width in mm*100 by the actual image width of the same segment. In this case (14mm*100) / 643 pixels = 2.177.

Resize your image using this ratio. The total image above is 1047 pixels wide, so 1047*2.177 = 2279 pixels wide. Once resized, you need to convert the image to a 1-bit bitmap image and save is somewhere. (As a sanity check before saying, this conversion should give you ~1400 pixels across the 14mm section for 1 pixel = 0.01mm).

Again, the way you do this will change from one image editing program to the next, but any decent image editor should support this. You should end up with a bitmap image like this: