To realize wireless communications in tera-hertz (THz) spectrum, the design of transmit signal with low peak-to-average power ratio (PAPR) is essential due to poor power amplifier (PA) efficiency in this extremely high frequency band. Although the discrete Fourier transform (DFT) spread orthogonal frequency-division multiplexing (DFT-s-OFDM) is a viable solution to reduce the PAPR than the conventional orthogonal frequency-division multiple access (OFDMA), the choice between these two schemes in the downlink reveals a fundamental tradeoff between PAPR and multiuser diversity. In this paper, to facilitate the optimal balancing of this trade-off, we propose a new downlink transmission scheme suitable for THz communications, namely DFT-precoded OFDMA with variable number of DFT-precoding chunks, where a base station (BS) is able to opportunistically vary the number of chunks to generate a multiuser downlink transmit signal. For the proposed scheme, we provide solid analysis on the PAPR of BS transmit signal with respect to the number of chunks, which enables the optimal design of various downlink transmission scenarios by opportunistic control of the number of chunks according to the required level of PAPR and multiuser diversity. The proposed scheme and analysis provide useful insights in the optimal design of future wireless systems, especially in THz bands with limited PA efficiency.