I believe the PGBC is technically operated in the style of an "insurance" pool, where recurring small fees charged to all eligible participant companies are intended to maintain the solvency of the fund over time. Whether that long-term vision will actually work, given the present cascade of pension plan abandonments, is anyone's guess.
The US has struggled to establish and maintain methods for funding basic 'life-support' pensions for the general working populace since the early 1930's. That system is under stress because of the pig-in-python effect of the post ww2 population surge and the longevity effects of improving sanitation and health care.
'Private pensions', operating supplemental to but independent of Social Security, have been viewed as a form of incentive compensation for the convenience of the employer's business goals. I remember very well hearing impassioned pleas for "portable pensions" in the early 1970's in the airlines and other 'growth' industries. Employers methodically used pension vesting as the principal method for locking-in key employees, so the owners opposed any plans for immediate incremental vesting and portability - leading to the wing-and-prayer system that now is failing so badly in the airline industry.
No compelling standards existed for pension administration and funding practices until modifications were made to the ERISSA rules (about 1987), with phased-in 'effective' dates in the early and mid 90's. Those rules required that pension promises be funded to some percentage of reality and the difference carried on company balance sheets as a liability. This was a sort of poison for companies that had methodically fudged the numbers, so companies began dumping the obligations any way they could, with cash buyouts and phenomenal projections re future rates of return being the main methods. Geniuses in the style of Michael Miliken and Ivan Boesky developed swift methods for wholesale cashectomy of companies that had any big money in reserve near the surface, however, so pension accounts became increasingly difficult in every regard. Companies in the US with consistent growth and profits are the only ones that have been able to sail through these vicissitudes with their pension promises intact.
The pain of all this as it plays out is palpable. It has the qualities of an inexorably unfolding Greek tragedy writ on the path that led us here. It follows, like night from day, from the willingness of employees, unions, and managers to rely on fluff and dreams about a prosperous future - instead of working with a less inspiring but more viable vision based on cold hard cash for modest but real pension accruals on deposit with independent fiduciaries.