how to indicate citations from the same, long document at different pages
When citing a specific page or pages of a longer document (not generally necessary for published journal articles although that may also be mandatory in some fields) one adds the page number to the reference. In the Harvard system one would write "Smith (1969, p. 24)" or "(Smith, 1969, p. 25)". There are as far as I know no hard rules when such references should be made but an author should consider the readers of the work they produce and facilitate finding the information cited as well as possible. In a short article of 10-15 pages this would not be difficult but in longer works one should consider providing pages. The point of references is after all to provide sources for information used in the article and one should not have to read an entire book to find it.

If I understand the second question correctly, citing different pages in a specific document is done as above and the reference in the reference list will be just the document itself; no need to refer to page numbers there. The reference list provides the litterature to find and the in-text referenes will provide information on where in the literature you need to look.