doc-view: resizing & scrolling

[knubee]

Doc-view is wonderful! Nice work.

One set of issues that arise is when we use it to view pdfs in one
window -- while making notes in another.

- if we have windows side by side, would be nice to be able to shrink
the size of pdf (or to scroll sideways within it)

- if we have windows above each other, M-PgDn ("page down" button)
will move through one window of pdf text at a time, but will fail at
the end of a pdf page (e.g., it will show emacs buffer with something
like "Page 2 of 15.")

thanks.