Short answer
On a Mac you can use Preview (select images → File → Print → Save as PDF) or, for combining many images with a chosen page size, use this browser converter — it runs locally in Safari or Chrome and never uploads your files.
Preview and Finder's Quick Actions can create a PDF from selected images, which is great for a quick one-off. This tool is handy when you want consistent A4/Letter pages, a specific order, or a borderless fit-to-image PDF, all without opening an app.
Everything is processed in the browser, so it's a safe way to turn a scanned ID or a signed contract photo into a PDF without it leaving your Mac.
Open the image in Preview, choose File → Export → PDF, or select several images in Finder and use the 'Create PDF' Quick Action. This browser tool is an alternative that adds page-size control and easy multi-image ordering.
Yes — this converter runs in your browser, so there's nothing to download and nothing is uploaded.