How do I invert colors on document

Adobe Express , both the mobile app and through online services, has many features for free that allows you to do that, including removing the entire background and adding and saving the image file with a transparent background.

But you are dealing with scanned images.

Scanned images involve many digital layers that are composed  of many other different types of data to make it appear as an single  image file readable to the human eye.

In which, case you should not confuse interchangeably Adobe Acrobat Pro (which is a PDF editing program) with a program that specializes in graphical image manipulation, such as Adobe Photoshop (as correctly suggested @try67), to include others like Adobe Illustrator, Adobe Express, Microsoft Paint, Inkscape, GIMP, for example.

Scanned images should be edited and tested externally before you import them in Adobe Acrobat or you will find yourself in a world of doom for using the incorrect program.

However, Adobe Acrobat do have the Preflight tool which comprises a more advanced set of tools to work around similar issues under the Print Production tool.

The Print Production tool  is already shipped with your paid subscription of Adobe Acrobat Pro (full desktop version).

Also, I am not sure why are you using the recommended guidance of the Accessibility tool when that feature is not meant to handle scanned images at all.

In any case, if you are copying and pasting these scanned images from a web browser (or any other program for that matter) directly onto a PDF document that is viewed in Acrobat, that is also the wrong approach (unless of course, you've taken the time to edit these scanned images and test them externally with another image editing software as suggested earlier).

All said,  the easiest way that you should've explored first,  is to employ the Scan & OCR tool => Insert => "From File..." to perform text and image recognizition as it uploads the entire scanned image to your PDF document and while retaining the page dimensions that are already set in that PDF document.

Then open the Edit PDF tool and you'll be able to see which bounding boxes pertain to text or image content.

Right-clicking with the mouse pointing device on an image bounding box will allow you to select "Delete" from the context menu, thus removing that image's colored background that appears behind the layered text bounding boxes.

OR,

You can also select from that same context menu "Replace Image", in which case you can upload an solid color  .png image file which will do the trick as changing the background color.