PNG to pdf

You'll need a 3rd party app from the App Store for that. https://apps.apple.com/us/app/photo-to-pdf-image-converter/id1534680120

Or use the Print option and save it as a PDF.

1. Open the image
2. Tap on the share icon (arrow coming out of a box)
3. Then look for there Print option.
4. In the Print dialog, pinch to zoom into the preview.
5. Tap again on the share icon or on the downward arrow next to the file name, and there should be a PDF option there to save as.

There's no reason to do so. A PDF is a container file that can hold multiple file types. All you'd be doing is putting a PNG within a PDF.