How to remove the grey watermark?
In order to remove the watermark, you can use a custom Image Enhancement Profile to load an image with altered contrast for example. It will allow you to remove the grey watermark but black letters will be available for recognition. Please see the example:
With standard profile the watermark was automatically removed, but some zones could be still corrupted and the value will be defined incorrectly due to the watermark artifacts (PO Number is recognized incorrectly):
Let's create Image Enhancement Profile (Project > Project Properties > Image Enhancement Profile > New...) with a contrast of 90. Thus, the watermarks are almost invisible, but the text itself is fine recognizable:
Then, we should apply this profile for all new images loaded ("high contrast profile" is the name of my profile):
If the mentioned pdf is loaded with this profile, the results differ, now the PO Number value recognized correctly: