This paper proposes a novel secure downlink framework that integrates Partial Non-Orthogonal Multiple Access (PNOMA) with short-packet communications (SPC) under keyhole fading channels, tailored for ultra-reliable low-latency (URLLC) services. Unlike prior studies that addressed NOMA, SPC, or keyhole effects in isolation, our work is the first to jointly consider all three aspects in a unified design. Closed-form expressions for the average secure block error rate (SBLER) and block error rate (BLER) are derived under both partial and full transmission information (PTI/FTI) assumptions at the eavesdropper, together with asymptotic analysis capturing the impact of blocklength, power allocation, and keyhole severity. Numerical simulations confirm that the proposed PNOMA-SPC system consistently outperforms conventional NOMA scheme in terms of latency, reliability, and secrecy, even under strong eavesdropping conditions. These contributions provide new theoretical and practical insights into the secure design of multiple access schemes for next-generation 6G URLLC scenarios.