The method involves determining printing conditions of a document/product, where the document is of analog, digital or passive type. Physical characteristics e.g. size, of cells of a form are determined based on the conditions so that a proportion of the printed cells showing a printing error that is due to printing irregularities, is higher/lower than respective preset values. Digital/text information is represented by varying appearance of the cells having the characteristics. The form is printed by applying the conditions, where the form detects a document copy that modifies the appearance. An independent claim is also included for a document securing device comprising a determination unit.