Privacy-preserving data collection is the act of guaranteeing the privacy of IoT information during the data aggregation process. Previously, several solutions have been suggested to guarantee privacy-preserving data collection in the IoT domain. The previous works can be summarized on three aspects such as (i) preserving the user privacy using data aggregation schemes, (ii) protecting the user raw data using encryption schemes, and (iii) protecting the user identity using blockchain technology. Several challenges remain to be solved to meet the requirements of the privacy-preserving issue. First, a single point of trust problem needs to be eliminated using distributed data storage. Second, end-to-end encryption should solve the raw data disclosure issue for all raw data given by the smart devices to a trusted party need to be aggregated. Third, both anonymity and pseudonymity disguise the identity of IoT devices; thus, there is no similarity between real identity and the used pseudonyms in the IoT network.