for generic PDF questions, the PDF specification has the best answers. The PDF 32000-1:2008 specification is freely downloadable.
The origin is by default the lower left corner of a page, with x going to the right, y going up.
But: There are a lot of way to make life more interesting. See chapter "14.11.2 Page Boundaries”.
> On 26. 02 2019, at 10:24, Евгений Король <[EMAIL PROTECTED]> wrote: > > Hello. How can i know where is origin of coordinate system. Where is point > 0, 0 on page. When it is different orientations of page it is different > location of 0, 0 point of page.