There is no concept of PNG, TIFF, GIF or other image formats in PDF. JPEG
is an exception, and in most cases you can embed a JPEG image directly into
your PDF file without having to uncompress and re-compress it (this would
result in more compression artifacts). To insert any other image format,
you need to extract the bitmap information from the source format (e.g.
PNG), and then use that bitmap (together with information about size, color
space, ...) to create your image in the PDF.
Take a look at the PDF specification, and read the information about images
and how they are stored. It is impossible to use the Acrobat SDK without a
good understanding of the PDF specification (which is included in the SDK's
documentation hierarchy).
Karl Heinz Kremer
PDF Acrobatics Without a Net
PDF Software Development, Training and More...