I am fascinated by the history of the history of writing implements. Here is one site that affords a glimpse at some of the materials Romans used for portable documents: Vindolanda Tablets.
From the Website:
"The Vindolanda writing tablets, written in ink on post-card sized sheets of wood, have been excavated at the fort of Vindolanda, immediately south of Hadrian’s Wall in northern England ... They were written by and for soldiers, merchants, women and slaves."
"Until the discovery of the Vindolanda ink tablets" it had been assumed "that wooden tablets written with a stylus were the commonest type of portable document. The discovery of ink 'leaf tablets' at Vindolanda was an enormous surprise to scholars."
But wooden tablets were by no means the only medium: "Perhaps the most familiar writing material is papyrus. Far more papyri have survived than any other category of document: they have been found in their hundreds of thousands in Egypt and the eastern Mediterranean. Another form of portable document was the ostrakon (plural ostraka), substantial fragments of pots which were re-used for writing in ink."