Thanks! In the Android version, i solved the problem of getting coordinates relative to the PDF page through recording the vpage measurement on OnPDFPageDisplayed(Canvas canvas, PDFVPage vpage) event. From the data of VX, VY, Height and Width of vpage, i calculate the zoom ratio and position of the center of the page and therefore i get the coordinates.
I am trying the same thing on the WinRT version but not successful. m_h, m_x and m_y of the vpage stay the same during zooming, while m_w changes. Could it be a bug? If this could work, it would solve my problem of determining the coordinates.