Dear Jack @jkallmeyer,
You can open the PDF file with Photoshop, a window will pop up asking you what page you want to open, select the PDF with the scale bars, it will open in PS; now you can select the scale bar you want with the rectangular marquee tool, press command c, then command n, enter, command v, save that frame on your desktop with command s and name it as you wish.
Then you will have the selected scale bar in a separate frame and can use it for all your images as drag and drop.
I hope that helps!
all the best,