Short answer
On Android, open this page in Chrome, tap to pick your JPG or PNG images from Photos or Downloads, choose a page size, and download the combined PDF to your device — no app, and nothing is uploaded.
Android has no consistent built-in JPG-to-PDF button across phone brands, so a browser tool is the most reliable option that works the same on Samsung, Pixel, Xiaomi, and the rest. You can select several images at once and they become pages in one PDF.
The file is generated locally, so it's fast even on a slow connection and your images never leave the phone.
You don't need an app — a browser-based converter avoids ads, permissions, and uploads entirely. If you prefer an app, look for one that processes files on-device; many free apps upload your images to a server.
Yes. Select all the images at once and they'll be placed into a single PDF in the order you add them.