What an HEIC file actually is
HEIC is a HEIF container holding HEVC-compressed stills. It stores roughly the same visible quality as JPEG in about half the space, and it can hold extras a JPEG cannot: depth maps, edit history, and the paired frames behind Live Photos.
Every iPhone since iOS 11 in 2017 shoots HEIC by default. Files reach other people through AirDrop, direct copies off the device, or an iCloud download that skips Apple's automatic conversion — which is why the problem usually appears at the moment of sharing.
What you get from PNG
PNG output is lossless, so the saved pixels are exactly the pixels that went in and re-saving costs nothing in quality. It keeps a full 8-bit alpha channel, which means soft shadows and anti-aliased edges survive against any background.
Universal since the early 2000s, and the format every design tool, browser and operating system handles without argument. It is also the safest format to hand to a printer or a colleague when you do not know their software.
What changes when you convert HEIC to PNG
PNG is lossless, so the conversion preserves exactly what the HEIC decoded to — but not the detail the HEIC had already discarded. You are freezing the current state, not recovering the original, and a common misunderstanding is that saving to PNG somehow restores quality. Expect the file to grow, often several times over, because lossless compression cannot match what lossy compression already did.
There is no transparency to preserve — HEIC files do not carry an alpha channel, so every pixel is opaque to begin with. Converting to a format that supports transparency does not create any; it only means you could add it later in an editor.
Going from HEIC to PNG keeps you in modern-codec territory rather than falling back to JPEG, so file sizes stay closer to what the iPhone produced. The trade is where the result can be used: HEIC fails almost everywhere outside Apple, and PNG succeeds almost everywhere inside a browser — but neither is a safe bet for someone else's desktop software, which is what JPG is for.