I have a small image, containing a number and a letter, the latter is in negative". How to invert?

I think if you zoom in on the  "A" you will see how poor the image quality is.  However, if all you documents are the same size and the letter location is in the same location in all your images.  You should be able to record an action that will invert the characters with better image quality then it currently has.   I have recorded such an action. To show how one may go about it.  I singled step through the action so you can see how it works and see the quality of the conversion.  All that is missing from the action is a final deselect step.  I can not post the action .atn file for the selections in it need to be relative to your document size.

Hi GabrielAl,

Image > Adjustments > Invert would work for this task.

You should record and action so you can run a batch action to all the images.

Here's an explainer on Actions in  Ps

https://helpx.adobe.com/photoshop/using/creating-actions.html

hth

.mj