Changing document colours

Hi,

I am partially sighted and can't read black text on white background.  I am trying to scan pages from a book to get them into pdf format so that I can use the Preferences > Accessibility > Documents Colours Options > Replace Document Colours faciltiy on Acrobat Reader.  The scanner I use generates jpg files and can export them to pdfs.  However I cannot change the colour of the exported files.  I have tried exporting in various types of pdf (pdf, pdfA, searchable pdf, searchable pdfA) but the colour change faciltiy doesn't work.

Can anyone suggest a solution?  I see the scanner can also export in tiff format - if that woudl make any difference.

Exporting as a ocr'd word file doesn't work as the text I want to read is an engineering textbook with lots of odd letters and diagrams and they just don't ocr properly.

Many thanks in advance