Short answer
You can convert a transparent PNG to PDF here, but note: PDF pages aren't transparent, so transparent areas are flattened onto a white background. The conversion runs in your browser with no upload.
This is expected behavior for every PNG-to-PDF tool — PDF has no page-level alpha channel, so transparency becomes white. If you need to keep transparency, a PDF isn't the right container; keep the PNG or use a format that supports alpha.
For logos or graphics you just want to share or print, flattening to white is usually exactly what you want.
No — transparent areas are flattened to white, because PDF pages can't be transparent.
No — the PNG is embedded losslessly; only transparency is flattened.