PNG to PDF can convert your png files uploaded from your device or from Google drive™ to common pdf file.
If you already upload png files, please wait a little for convert it to pdf. 
When conversion of file is done you can download it as pdf.
After all files conversion you can download common pdf file. 
You can use multiple png files for conversion so, in output you will have 1 pdf downloaded per png uploaded and 1 common pdf file with all png images inside.