PNG PNG preserves the PDF page background as transparent if the source page has no fill (rare). JPG PNG cannot store transparency — pages render onto a white background. For transparent output target PNG, TIFF, or WebP.
You're rasterizing vector content into pixels — zoom amplifies the rasterization. If you need infinite-zoom output, target SVG (where the PDF contains vectors); for sharper rasters, bump DPI to 300 or 600 in advanced options.