You are almost certainly seeing references to ImageMagick, which has a "convert" utility that potentially allows .png to .pdf conversion e.g.
convert image1.png image2.png image3.png output.pdf
You will need to install ImageMagick to use its "convert" utility (make sure it's properly added to your Windows PATH).
I am not personally familiar with PDFTK (so I could be wrong) but I get the impression it is primarily used in conjunction with ImageMagick because it does splitting and merging (not conversion).