Stephen Hawking and his dissertation are high-profile as these things go. The NPR mentions other popular items generating 100s of requests per month. I've run across items with lifetime request counts in the double or triple digits frequently (and suspect I doubled the count on one particular item).
More often, though, the truth is that this material simply isn't available online. There are several thesis repositories (either Michigan State or University of Michigan are one, as I recall), and I can frequently turn up a shelf reference via WorldCat ... somewhere.
But there's work from surprisingly prominent names in numerous fields that simply isn't available in electronic format. The worst case is for materials from rougly 1924 - 1980: to late to be out of copyright, and too early to have been composed, or converted to, digital formats (and 1980 is an early cut-off date for that, though it's when material seems to start appearing in bulk).
This includes PhD dissertations, Masters theses, and numerous academic or other writings, often including government documents not under copyright. Thankfully with Sci-Hub, actual published academic journal articles can be found, freely, with a very high success rate. Particularly painful for me are popular magazine and newspaper items, for which even the indices are very frequently locked behind site-restricted or affiliate-only access.
The time-and-effort differential of being able to look something up online, vs. travelling many miles to a facility for access, is tremendous. And it absolutely stops a great many incidential queries dead.
See Rick Falkvinge's excellent rant about how the KRACK vulnerability was blocked behind corporate-only paywalls for over a decade:
https://www.privateinternetaccess.com/blog/2017/10/the-recen...
Note that the issues here are twofold. One element is the task of scanning and making available documents, and organising the results in a manner useful for search.
But much the harm is the direct consequence of the present regime of copyright and paid access to information, AS WELL AS the perverse incentives of advertising-backed media and media manipulation have created a media regime that is actively harmful to society.
I'd really like to see the elements of this addressed.