A quick tip for command line users: if you want to convert .png files to .pdf:
convert *.png filename.pdf
Simple! If you want to keep control of page sequence or specific files to convert:
convert page1.png page2.png page3.png filename.pdf
Needless to say, but I’ll say it anyways, the .pdf won’t be in vector graphics, so not scalable but that is because .png is not a vector graphics format.