JPEG (or JPG) is a graphics file format designed for compressing and saving photos. It is excellent for the photograph of a case head, for example.
The JPEG compressing algorithm creates so called “artefacts”, which are small rectangular speckles. These normally do no harm in a photograph of a natural object. But they are very bad for texts and/or drawings, because they destroy the sharp outline of letters, figures and lines.
So, if you are scanning/photographing:
- books with mostly text, line drawings and b/w photographs,
- engineering drawings
or the like, please avoid saving the result in a JPEG file. I personally prefer PNG. An alternative is TIFF. Those who try to read the texts you shared will be thankful.