Visio Shape object have ID and UniqueID properties.
ID is a counter, UniqueID is a GUID.
Visio Page have only ID propery.
Is it a some GUID for Page, auto generated by Visio by default (because my app can't modify the document)?
Sort of. Try:
page.PageSheet.UniqueID(visGetOrMakeGUID)
It's like the one you have for shapes. But for pages. See more details on UniqueID
here