As the log book is a legal document it is required to be properly bound and pages are numbered accordingly.
Your local CAA regulations should tell you how long they should be kept'
As to electronic documents, they should have some sort of "blocking devince" contained within them to prevent any manipulation of past entries. It should be a live document, ie any entry made will automatically include the time and any amendment required to be made to a previous entry will require the person to enter a password to edit the document.