I am far from expert in this, however my belief is that the choice of DCF vs PCF is not strictly about ad-hoc vs infrastructure, but that PCF inherently can only work in infrastructure mode as there has to be a coordinator, while DCF could be used ad-hoc or infrastructure; also was the only mandatory method.
DCF and PCF have been superseded by new Hybrid Coordination Function in 802.11e. (Wikipedia)
Following note says Cisco didn't implement PCF, and that 802.11e was going to effectively kill it off. You can still find academic papers though. Here's one devising distributed hybrid coordination functions, designed to be PCF-for-ad-hoc, but only done in simulation.
Nice summary presentation of QoS techniques for wifi here
In 2002, Cisco forums included the following post from staff, (emphasis added):
From: bmcmurdo Cisco Employee
09-04-2002 09:07 AM
Re: PCF or just DCF in Cisco AP ?
PCF is not implemented on Cisco APs. PCF is not generally thought to be a very effective/useful QoS technique
The radio Access Point (and radio firmware) are "PCF aware" meaning if they
detect a PCF coordinator they will "play nice" in that area and back off a
little so it can do it's best effort.
IEEE 102.11e [sic] working group is currently working on a QoS standard but that has not yet been ratified.
802.11e came out in 2005, but has since been superseded.
This blog post from a CCNA-Wireless suggests that you have to hunt to find PCF in real life: "curious has anyone else seen this or PCF in the wild ?"