Most PDF documents have pages where the text occurs in specific rectangular blocks. If the blocks are in the exact same location (even if they alternate on odd/even pages) then it would be fairly easy to create a macro that takes a screenshot of the text block, sends that image to an OCR engine, saves the text, and then goes to the next page.
I've done this many times. In fact I've done multiple 300 page books this way, (actual physical/paper books) long before KM even existed. Doing it on a PDF-formatted book would be a piece of cake.
There are even easier solutions if the number of pages is small, say under 50. You could have a macro that simply gets triggered any time the system clipboard changes. When it changes, the macro will check that the clipboard contains an image, perform OCR on that image, and append the resulting text to a KM variable. That macro would probably contains only two actions. All the user would have to do is select each page manually with the mouse using the screen capture shortcut.