I don't believe any such animal exists.
Any online tool would have to decompile the the PDF structure, not simply read it. I've never seen any browser-oriented tool which can alter or even display internal PDF data. Not even the simple internal data such as dimensions, colorspace, etc.
Even things such as Kodak's print production PDF processor doesn't display enough data to know what is vector and what is raster.
To answer the comment above.
There are actually multiple layers to any PDF in my understanding.
There's an underlying construction layer which contains as much native application data as possible. There's an object appearance layer which contains location and raster appearance of all objects. And then there's the master rendering layer which contains a preview the entire page as a raster image (resolution dependent upon PDF job options.) Whether or not these layers exist depends greatly on the PDF job options and the generating application, as well as any "maintain application editing ability" checkbox.
When you open a PDF with Illustrator, Illustrator reads any native application construction data it can, then constructs objects. If no native application data can be used, Illustrator reads the object appearance data and displays that (a raster image). And as a fallback, if neither of the two previous items are present, Illustrator displays the entire page as a raster image.
Any online viewer is merely reading the master rendering data and displaying that. Online viewers don't customarily even bother looking any deeper. They are focused on displaying entire pages without any need to alter, view, or examine the individual elements within those pages.
Online viewers are similar to how Photoshop reads PDFs. Photoshop reads the master rendering data by default. Which is why every PDF opened in Photoshop will rasterize the entire image. The only caveat with Photoshop being the fact that if Photoshop sees Photoshop construction data it will use that data first.
This is merely my understanding of the PDF architecture. I could be incorrect.