In Google's documentation, they say for an impression to occur the ad has to actually be shown on the page. In your experience, have you received impressions for ads that are buried beyond the first page & the user only views the first page?
Am I right in saying that an Ad can appear on a page but be at the bottom of the page and even if the use doesn't scroll down to see it, it's counted as an impression?
You are correct. It just has to load. It doesn't matter if they scroll down. There's not really a technical way that I can think of to track that.
It makes the clickthrough rate a bit misleading if you're appearing around position 10 (since the advert sometimes appears at the bottom of page 1 and sometimes at the top of page 2).
I often bid a higher that i really like to e.g .10cents a click and then slowly lower my bid down to what I prefer to pay over the course of a few days and sometimes i have managed to stay at a certian position because of getting a good CTR on certain ads.