Find dpi for rectangle coordinates in A4 format

Hi,
I need to create some text boxes (form fields) inside a blank page, If I use the method :
myPage.AddFieldEditbox(rect, "fieldName", false, false);
I need to provide a rectangle with sizes in pizels (left,top,right,bottom) but I don't know which values to use.
I have an A4 paper format but I would also need the dpi in order to compute the pixels, how can i get the DPI of the page?
Thank you

dear user:
the PDF coordinate system is always in 72dpi,
that mean A4 paper has page size: [210 * 72 / 25.4, 297 * 72 / 25.4].
and PDF using math coordinate system, that mean: coordinate of left-bottom is [0, 0]
and left-top is[0, 297 * 72 / 25.4]
all line width and text size are all using 72 dpi.