pnmtoddif: add page (#11753)

pages/common/pnmtoddif.md

@@ -0,0 +1,12 @@
+# pnmtoddif
+
+> Convert a PNM image to a DDIF image file.
+> More information: <https://netpbm.sourceforge.net/doc/pnmtoddif.html>.
+
+- Convert a PNM image to a DDIF image file:
+
+`pnmtoddif {{path/to/image.pnm}} > {{path/to/image.ddif}}`
+
+- Explicitly specify the horizontal and vertical resolution of the output image:
+
+`pnmtoddif -resolution {{horizontal_dpi}} {{vertical_dpi}} {{path/to/image.pnm}} > {{path/to/image.ddif}}`