I am not aware of any method that lets you pick the quality of PDF export from Pages.
Pages puts out a fairly high quality PDF, so I'm guessing what you wan to do is lower the quality and save on file size. I can think of only two ways to to this.
One: the method I use is to send the file to my Mac, where I can use one of several filters in the Preview app to reduce the size/quality of the PDF. There should be similar apps for Windows.
Two: find an iPad app that can reduce PDF quality size, then use the Open In options from Pages to copy the PDF directly to that app. Neither of my PDF apps (GoodReader and PDF Expert) can do this, and I have not heard of one; but I don't see any reason why such an app can't exist.
Both of the apps I have can Zip (compress) files; so if it's a matter of sending a smaller file over email or some other method, that is doable. Hold in mind that PDF files that contain a lot jpg images wont' compress much; since the image is already compressed.
In cases like that, it is better to reduce the size/quality of the images before importing them into Pages; rather then depending on the sizing and masking in the app. Pages does not alter the original image when editing, so the entire file ends up in the export; no mater how little is visible in the document.
I hope this helped, at least a little bit.
Good luck.