What conversion does

The browser decodes the pixels and writes a new file. The original stays on disk. JPEG and WebP get a high-quality encode. PNG is lossless. Images to PDF stacks each photo as a page. PDF to JPG paints each page as a picture. Metadata does not copy across.

Need it smaller, not just a different box? Compress images. Need GPS gone and the same pixels? Remove photo metadata. Longer version: convert without uploading.