Most people never choose a photo format. The phone chooses for them. On an iPhone that means HEIC, and it stays HEIC until something downstream complains, like a Windows laptop or an upload form that only takes JPG.
Picking the right format before you hit send saves a lot of back and forth. Each of the three has its own job. If you want to learn more about the settings that change quality and file size, this guide goes through them one at a time. It also has conversion steps for every device.
Keep your originals in HEIC
HEIC is a HEIF container holding images compressed with HEVC. Apple switched iPhones to it with iOS 11 in 2017, mainly for efficiency. According to Apple's WebKit team (the people behind Safari's engine), a HEIC photo needs about half the space of a JPEG that looks the same.
For a photo library that lives on Apple devices, that's a good deal, and there's no reason to convert your originals.
The catch is compatibility. Windows needs two Microsoft Store extensions to open HEIC files, one free and one priced at $0.99. On the web, Chrome and Edge still don't display HEIC images, while Safari has supported them since version 17.
Send a single photo as JPEG
If someone just needs to look at a picture, send a JPEG. Practically every device and app opens it, and most websites expect it.
Android has been able to decode HEIC since version 8.0, and Google Photos accepts the format. When you don't know what device or app is on the other end, though, JPEG is still the safer choice.
You can make an iPhone shoot JPEG from now on under Settings > Camera > Formats > Most Compatible. Or keep shooting HEIC and let the phone convert on export. In the Photos settings, set Transfer to Mac or PC to Automatic, and photos copied to a computer arrive as JPEG.
When a PDF is the right call
Documents are where PDF earns its keep: signed forms and receipts, or a rental application that runs to six pages. A PDF keeps the pages together and in order. It prints predictably, and most upload portals accept it. Print shops often ask for PDF too.
What changes when you convert?
Converting HEIC to PDF isn't a neutral copy. PDF has no way to store HEVC-compressed images, so every converter decodes the photo and re-encodes it, usually as JPEG.
Two things follow from that. The file usually gets bigger, not smaller. And the lost quality can't be recovered later, so the PDF should never replace the original.
Some converters strip metadata as well. That removes GPS location, which is good for privacy. It can also remove the color profile iPhones embed, and then colors look flat.
A rule of thumb
If I had to boil it down, I'd keep HEIC for the library and use JPEG when a single photo needs to travel. Anything that looks like paperwork gets a PDF.