I am looking to get the best pdfs for sheet music possible. I would be interested in hearing about software (free preferred), and techniques that will give superior results.

Looking for best way to go about scanning so that the copy can have a darker print than a light original, to clean up the background so it is “whiter”, remove blemishes, etc.

For sheets already in the pdf format am interested in the same as above along with being able to straighten a crooked image.

Also any other editing that would be beneficial such as typing in chords, blocking out incorrect chords and the like.

Any tips and techniques appreciated.