The method of marking paper documents consists of applying a colourless and semi-transparent marking element and a location element to the surface of the paper document by printing an identification code for identification by X-ray fluorescence. The marking element is prepared by mixing a colourless ink base with a coding mixture as a colloidal dispersion of at least two chemically inert compounds of lanthanides and / or actinoids and / or transition metals. The location element consists of a fluorescent-generating pigment when irradiated with infrared or laser. The invention also provides an identifier for marking and identifying paper documents, which is formed by a combination of the elements described above.