Palm leaf manuscripts, crafted from specially treated palm leaves, are invaluable historical documents. However, they degrade and tend to become brittle over time. To date, plant essential oils and glycerin are the used materials to improve the flexibility of palm leaf manuscripts, but the effective duration of these materials is short due to their volatility. This work introduces ionic liquids, a nonvolatile and stable material, to achieve durable toughening of the palm leaf manuscripts. We select 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIm][BF4]), one of the most used ionic liquids, as the subject of research. We find that [BMIm][BF4] does not alter the palm leaf manuscripts' appearance and significantly improves their tensile strength and fracture toughness by 39.9 and 101.0%, respectively, while reducing the bending modulus by 25.7% and increasing the bending fracture deflection by 2.6 times. Notably, [BMIm][BF4]'s toughening effect for palm leaf manuscripts is sustainable for over 2 months, outperforming traditional methods that last only about 1 week. Even under harsh conditions, such as low humidity, high temperatures, alkalinity, and UV exposure, [BMIm][BF4] still maintains its effectiveness. Through the analysis of its composition, structure, and theoretical simulation, we reveal that [BMIm][BF4] penetrates the manuscripts, filling the pores between the inner structure of them and adsorbing strongly with cellulose. This enhances load transfer and reduces the stress concentration, resulting in increased toughness. This research provides innovative materials for palm leaf manuscript conservation and deepens our understanding of their mechanical properties.