Sometimes your PDF viewing codes need to provide scroll capability, especially when the page is zooming into a large size exceeding the displayable area in the screen window. To scroll the page to a different position, horizontally or vertically, you can adjust the start_x and start_y parameters when you call FPDF_RenderPage function.
Because start_x and start_y parameters always specify the position of the top-left corner for displaying the page, so when you scroll down, the top-left corner of the page will go up, therefore the start_y parameter should be decreased. Sometimes the parameter can become negative, this is OK, this is a way to tell the FPDF_RenderPage function that the top-left corner of the page is not visible any more. Likewise, when you scroll up, the start_y will increase. When you scroll left/right, the start_x parameters will increase/decrease accordingly.
Here is an example how you can scroll the page vertically to half of its height:
// Calculate the new start_y, decreased by the scroll pixel size
start_y = 10 – page_height / 2;
FPDF_RenderPage(hDC, pdf_page, 10, start_y, size_x, size_y, 0, 0);