Strength and stiffness of structural materials and fibers is usually increased at the expense of strain at failure and toughness. Our recent analysis of size effects in individual electrospun polyacrylonitrile (PAN) nanofibers demonstrated unusual simultaneous increases in strength, stiffness, AND toughness with nanofiber diameter decrease. Ultrafine nanofibers with diameters in the range from 100-200 nanometers exhibited the highest mechanical properties. Structural investigations and comparisons with mechanical behavior of annealed nanofibers allowed us to attribute high ductility and toughness to low nanofiber crystallinity resulting from rapid solidification of ultrafine electrospun jets. Here we study possibility of further modification of the unusual mechanical behavior by adding plasticizer and changing solvent. Effects of plasticizer and solvent on average fiber diameter, PAN nanofiber crystallinity, and mechanical properties are examined. We show that replacing dimethylformamide (DMF) by dimethylacetamide (DMAc) as a solvent or adding small amount of ethylene carbonate (plasticizer) resulted in thinner, less crystalline nanofibers. The mechanical properties of these nanofibers in the intermediate (250-500 nm) diameter range significantly exceeded the mechanical properties of pristine DMF-spun nanofibers while maintaining simultaneously high strength, stiffness, and toughness. Correlation with nanofiber crystallinity supported the previously introduced structural mechanism of unusual mechanical behavior. The results open up new ways to tailor and further improve mechanical properties of nanofibers. Reported improvements in strength and toughness in the intermediate diameter range can lead to inexpensive simultaneously strong and tough composites for safety critical structural applications.