Para La Tierra is a research organisation rather than a tour operator, and it is the most interesting thing you can book in Pilar. It is based at the Centro IDEAL on Mariscal Estigarribia, works the Ñeembucú wetlands and the Atlantic forest, and keeps a museum and reference collection in town.
What a visitor can actually join is the three-week Methods in Primatology Field Course, run by the National Geographic Explorer Rebecca Smith across three habitats and two bases, finishing in Pilar with the town's urban howler monkeys. You learn VHF telemetry, camera trapping, blow darting, non-invasive faecal and urine sampling, distance sampling and habitat assessment, and you design and run your own small project and present it. €2,595 covers everything on the ground including the food and the transport between field sites.
If three weeks and that price are not what you are after, they also take research, museum and education interns, and volunteers, on shorter terms, and there is a primatology course you can do online first.
Be clear about what this is not: it is not a day trip, there is no drop-in visit, and there is no telephone number anywhere on their site. Everything is arranged by email.