Type3 fonts are the least useful type of fonts in PDF file specification (which is why most PDF creations tools no longer use them). Type3 fonts are generally used for symbol characters such as bullets or custom characters.
Type3 fonts offer poorer display quality (because they are bit mapped rather than vector graphics) and they have no Encoding. So there is no mapping table to show the character value for each glyph. If you have control over PDF creation, we recommend avoiding them and using Postscript/OpenType font options.
Because there is no Encoding table (tells us glyph 1 is A, glyph 2 is B, etc) and the font does not map onto any supported fonts in a browser, converting to HTML5 or SVG represents something of a challenge. So the software cannot convert them to text but renders them as a graphic on the display, preserving the display of the page.