a). Page: To choose page size here. You can choose one of the 30 standard sizes listed.
b). Position: You need to choose the position for the source image on the PDF page, unless the page size is the same with the original image size.
c). Margin: You need to choose the margin, unless the page size is equivalent to the original image size.
If you don't know how to use or choose the above options, please just use the default value or ignore them.