Short answer
On an iPhone you can turn photos into a PDF without installing anything: open this page in Safari, tap to choose the images from your Photos or Files, and download the combined PDF straight to Files. Everything happens on the phone — nothing is uploaded.
Apple has a built-in route too — in the Photos app, select your images, tap Share, then Print, then pinch-out on the preview to open it as a PDF and save it. That works but is fiddly and easy to miss. Using this converter in Safari is a two-tap alternative that also lets you set A4 or Letter page sizes and combine many photos into one PDF in a chosen order.
Because the conversion runs in the browser, it works the same on any iPhone model and any iOS version with a modern Safari, and it keeps sensitive images (an ID, a signed form) on your device.
Yes. This tool runs in Safari, so there's nothing to install. You can also use the Photos → Share → Print → pinch-to-PDF trick built into iOS, but the browser tool is quicker for combining multiple images and choosing a page size.
When you tap Download, iOS offers to save the PDF to the Files app (On My iPhone or iCloud Drive) or to share it directly to Mail, Messages, or another app.