Apple's Photos app is capable of more than just displaying images. It lets you edit, annotate, and print images directly from your iPhone. In addition, you can also use it to combine multiple images into a PDF document, thanks to the Print functionality.

To do this, make sure the images you want to combine into a PDF file are present in the Photos app and follow these steps:

1. Launch the Photos app on your iPhone.
2. Go to the album containing the images you want to combine into a PDF and choose the ones you want in the PDF. Make sure to select the pictures in the order you want them to appear in the PDF document.
3. Hit the Share icon in the bottom left and select Print from the iOS Share Sheet.
4. On the Print Options screen, tap the Paper Size option and select Photo Large.
5. Tap the Share icon again and select Save to Files.
6. Navigate to the folder where you want to save the PDF file in Files.
7. Tap on the name in the text box at the bottom of the screen and give a suitable name to the file.
8. Finally, hit Save to save the PDF file.