Neevia Document Converter eXpress is a very handy online tool for converting your documents to other file formats right on the browser. I tested and used it for converting PNGs to PDFs, and the conversion process was fast and completed without any problems. I used the "Wait for conversion in browser" option, but you can also specify an email address and have the link to the download page emailed to you instead. The file size limit for uploads is 1 MB.
The supported Output formats are:
- PDF/A
- PostScript
- EPS
- BMP
- JPEG
- TIFF
- PCX
- PNG
- GIF
Neevia Document Converter eXpress is a free online tool, but the company also offers paid desktop programs for creating and modifying PDFs.