3'-5' exonuclease that specifically cleaves precursor piRNAs (pre-piRNAs) at their 3' ends (PubMed:26919431). Trims pre-piRNAs to their mature size, a process required for piRNAs maturation and stabilization, and subsequent pre-piRNAs 2'-O-methylation (PubMed:26919431). The piRNA metabolic process mediates the repression of transposable elements during meiosis by forming complexes composed of piRNAs and Piwi proteins and govern the methylation and subsequent repression of transposons (PubMed:26919431). Interacts with Papi/Tdrkh; interaction takes place on the mitochondrial surface and recruits PNLDC1/trimmer to PIWI-bound pre-piRNAs. Belongs to the CAF1 family.