Im rendering a pdf using pdf js library. There I can specify zoom (scale) property. Which is fine. I can define pretty high zoom , let's say 8x and still get decent quality of the rendered pdf. However if I were to try to same pdf but converted to graphic image format like jpeg. And then try to render it with high zoom the quality is very bad. Why is that so?
You are describing the difference between vector graphics and raster graphics. A vector graphic format contains contains commands telling how to draw an image. A raster format is an array that tells what the color is at each position in the image.
PDF is largely a raster format (Yes, you can embed a raster image in a PDF). A PDF that has in instruction to draw a line or draw a character can be zoomed to any degree and the drawing will be correct.
In a raster format, if you zoom, eventually you see the individual pixels in the array and they cannot be zoomed any more without distortion. Text in a JPEG or PNG file becomes jagged as you zoom.
On the other hand, try to create a photographic quality image just with drawing commands and you would get huge files.