Android:
Using the Gallery or Photos app
1. Launch your Gallery or Photos App.
2. Open the photo you want to convert.
3. Click on the (three dots) icon to access more options.
4. Choose Print.
5. In the Printer Options, select Save as PDF.
6. You can set the Paper Size and Orientation.
Helpful video tutorial: https://www.youtube.com/watch?v=GpemaA646O0

Iphone:
Quick, easy steps for converting your JPGs to PDFs on iPhone.
1. Open your iPhone Photo app and find the photo you want to PDF.
2. Select the photo and click the Share icon.
3. Choose the Print option. …
4. Zoom in on your photo – this opens your image in a PDF viewer.
5. Tap the Share icon again once you're in the PDF viewer.
6. Save to files (Downloads, desktop etc.)
Helpful video tutorial: https://www.youtube.com/watch?v=KzrM31FHKAo