The sloooooow way is to simply begin scrolling page after page until I finally get there more than 200 pages into the document. Avoid the horrible scroll of death.
The FAST way is to use the search field!
Enter "Lesson 20" into the Search field. Make sure you include the quotation marks!
Then hit RETURN until you finally get to your desired page.
In this case, Lesson 20 finally began on page 274!!!
I got there in just a couple of clicks.
This will become an increasingly important skill as school districts gradually move to OER and other electronic and pdf-based curricula.