How to convert a photo to PDF
Every device can do this, in different ways and different places in the menus. Here is how on each, plus combining several photos into one file.
On an iPhone or iPad
From the Photos app: select the photo (or several, by tapping Select and choosing more than one), tap the share icon, and choose Print. In the print preview, pinch outward on the page thumbnail to open the full-page view, then tap the share icon again and Save to Files to save it as a PDF. Selecting multiple photos beforehand puts every one into the same PDF, one photo per page, in the order they were selected.
On Android
Google Drive has a built-in Scan feature (tap the + button, then Scan) that photographs a document directly into a PDF, with automatic cropping and contrast, which works well for documents rather than ordinary photos. For an existing photo already on the phone, the Google Photos app doesn't export to PDF directly; use the same print-preview trick as iPhone through the phone's Print option in the photo's share menu, or use a browser tool.
On Windows
Open the photo in the Photos app, or select it in File Explorer, and choose Print. Set the printer to Microsoft Print to PDF and print; this saves the image as a one-page PDF rather than sending it to a physical printer. This method only handles one photo per PDF at a time.
On a Mac
Open the photo in Preview, then File, Export As PDF. For several photos in one PDF, select them all in Preview's sidebar first (or select multiple files in Finder and open them together in Preview), then use File, Print, and in the print dialog's PDF menu choose Save as PDF; this combines every open photo into a single multi-page document.
Combining several photos with more control
Stayput's image to PDF tool (and the JPG to PDF preset) lets you drop in multiple photos, reorder them by dragging, and choose the page size and margins, all in the browser before generating the PDF. It handles JPG, PNG, WebP and HEIC photos straight from an iPhone camera roll, and nothing is uploaded to produce the file.
Questions
Can I combine several photos into one PDF?
Yes, on every platform above, either by selecting multiple photos before using the print-to-PDF trick, or with a tool built to combine several images into one document with control over the order.
What is the print-to-PDF trick and why does it work?
Printing normally sends a page to a printer; choosing a "Save as PDF" or "Print to PDF" destination instead captures that same rendered page as a PDF file rather than physical paper, which works for a photo just as well as a document.
Does converting a photo to PDF reduce its quality?
The print-based methods can slightly compress or resize the image to fit a standard page. A dedicated converter that embeds the image at its original resolution keeps it unchanged, just wrapped in a PDF page.
Can I scan a paper document into a PDF, not just convert an existing photo?
Yes: the iPhone's Notes app and Android's Google Drive both have a document-scanning mode that photographs and auto-crops a physical page directly into a PDF, which is more reliable for a document than photographing it manually.