What is the sub processor actually going to doing? Well it seems they have been tasked to "manage our (The PC's) personal data" ?
It might be helpful to note that "Processing" of personal data means any operation or set of operations performed upon personal data or sets of personal data, whether or not by automated means, such as collection, recording, organisation, structuring, storage, adaptation or alteration, retrieval, consultation, use, disclosure by transmission, dissemination or otherwise making available, alignment or combination, restriction, erasure or destruction.
So I would give thought to which of the above should be stipulated in the contract. Are the sub processors going to work in the PC's office (if they have one?) Erasure & Destruction in line with GDPR 2018 will be likely processing items, but I would be interested to know how they would erase personal data from the PC's ICT infrastructure, whatever that amounts to.