"Piezas gráficas de mano" is much broader than just flyers or handbills (volantes). It includes a whole range of printed material. The following are from Argentine sites:

"Aplicación: Uso de Procesador de texto para la diagramación de piezas gráficas de mano (volantes, folletos, cartillas, tarjetas, sobres, cartas y afichetas)."
http://clubensayos.com/Acontecimientos-Sociales/Comunicación...

"5.1.1 Encabezado y pie de página general para piezas gráficas de mano
Las presentes medidas son de aplicación en: volante, folleto desplegable y publicación tipo revista o brochure."
http://www.senasa.gov.ar/Archivos/File/File964-manlIdentidad...

It can apparently cover just about any printed matter short of actual books, but it clearly has connotations of publicity or promotional material, particularly in your context.

"Gráficas" really just means printed, I think (as in "artes gráficas", which is what printing companies are quite often called). But since it says "se imprimieron", you don't need to says "printed". I think it's enough just to say that "some 100 000 pieces of promotional material were printed". I think this term will make it clear enough that we're talking about the kind of items mentioned in my first source: handbills/flyers, brochures, leaflets, booklets, envelopes, cards, even posters. It's difficult to think of an English term that covers all this.

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