This unit is used to be device independent and uniform regardless of the current screen or device resolution (DPI).
A resolution in dots per inch (DPI) is needed to convert from document coordinates to pixel and vice versa. This is done using the following equation:
Pixels = (Document Units / 720) * Resolution
The DocumentToPixels method uses this exact equation to convert from document units to pixels.
The following formula is used to convert from pixels to document coordinates:
Document Units = (Pixels / Resolution) * 720
The PixelsToDocument method uses this exact equation to convert from pixels to document units.
When loading documents, each page size is stored in the Size property in document units. Although rare, some documents formats such as TIFF and PDF support having different size pages and thus, the size is stored in the page object itself and not in the document.
The global document resolution is parsed by the loader and set in the DocumentPages.DefaultResolution value. Again, each page maybe have different resolution depending on the document format and hence, DocumentPage.Resolution will contain the actual resolution of the page (in most cases, equal to DocumentPages.DefaultResolution).
Most of the coordinates uses values of LeadPointD, LeadSizeD and LeadRectD that represent a point, a size and a rectangle area. The Document class contains the following helper methods to quickly convert any of these values from document to pixel coordinates and vice versa: Document.PointToPixels, Document.PointToDocument, Document.SizeToPixels, Document.SizeToDocument, Document.RectToPixels and Document.RectToDocument.