PDFs might still work - but will the storage devices they are on? There are good reasons to doubt this: tapes, CDs and DVDs age and degrade over time; plus devices to read them only last so long, and may not be produced anymore at some point. Classic "spinning" hard drives degrade, too; and even ignoring this, will there still be conputers supporting, say, SATA, in 50 years?

And then there is a trend to switch to onboard soldered flash in new devices which adds further problems.

Not saying this is impossible to overcome, but "PDF might still work" is at best solving a part of the problem, for only a part of all data (PDF is great - but only for some types of data) one might want to preserve.

The cloud-based storage will never die. (And if it does, we'll have bigger problems on our hands than lost PDFs.)