Use the Photos app
Step 1: Launch the Photos app on your iPhone and select the photo you want to convert to a PDF file.
Step 2: Hit the Share icon at the bottom of the screen.
Step 3: Tap on Print.
Step 4: Hit Print at the top right corner of the screen.
Step 5: Select Save to Files from the pop-up menu.
The photo will be saved as a PDF copy on your iPhone.
To know other ways to convert a photo to pdf on iphone click here.