Sept 16, 2026.
(This may be well known, but I just discovered it yesterday evening.)
If an html page has a link to a PDF file,
e.g., wherever/documentname.pdf, then someone clicking
on that link will be brought to the first page of the document.
(Unless he or she has visited it before, in which case I think
he or she is brought to the page he or she most recently left.)
But one may want one's link to take the viewer to a particular
page of the document; say page 3.
In that case, one should insert #page=3 at the
end of the URL.
E.g., expand the URL shown above to
wherever/documentname.pdf#page=3.
(The one other function I know of the character # in URLs,
which I have used for years, and assume is well-known,
is in linking the user to a particular spot on an html page.
E.g., if you want to put in a link saying "You can find
the solutions here" to a list headed "Answers" on a page
.../discusssion.html, you might mark the word "Answers"
that you want to guide the reader to on that page
as <a name=solns>Answers</a>, and enter
the sentence linking to it as "You can find the solutions
<a href=".../discussion.html#solns">here</a>".
But if you don't own the page that you want to link to a point of, and
the owner thereof doesn't happen to have put
a <a name=...> marker at the place in
question, then I don't know any way to create such a link.)