How to lighten converted grayscale images in Acrobat Pro DC?

If you have Adobe Photoshop installed on your system, you can go to the Edit PDf tools in Acrobat, right click on an image, and choose Edit Using... and choose Photoshop (or another image-editing application).  Once you have adjusted the image in Photoshop, exit, and it will ask if you want to save your changes.  Reply Yes, and the tweaked image will be shown in your PDF.

I hope this is useful for your circumstances.

My best,

Dave