I know why.
They are a few reasons why you can have can blurry text. The last one, is what concern you in your particular case.
-> Corrupted or forgotten to embed font. PDF needs to be remade properly.
-> Poorly scan'ed image, so text is not sharp. It's image, nothing can be done.
-> GPU rendering. huh what? you say. Modern UI apps are FULLY GPU rendered. This is in reality what we want... the only reason why we ever had CPU rendered apps was because GPUs were not powerful enough back in the very very old days, non programmable, and very costly... so as we assume lowest common denominator when designing a program, the CPU was decided to be used to render things. In fact the entire OS GUI was designed on the CPU. CPU sucked awfully at rendering image. It was never designed to do such a role, but it was done none the less, as it was the best.
If you want to have fun, go on an XP machine, put a nice large background, take a window, and drag it around the screen continuously. Beside the fact that the mouse/window position are not in sync as you move your mouse (one of them plays catch-up in drawing the new position), but also one of your CPU cores will spike at ~90-100%, despite a Core i7 inside. Remember when Vista came out? OEMs (Dell, HP, etc..) put Vista on underpowered system which resulted in poor performance. If you had a high-end gaming PC (true dual core, 64-bit, 3-4GB of RAM), Vista 64-bit ran smooth as butter.
The reason, Aero... a first step for Microsoft to change the GUI of the OS to GPU rendered. If you had a dedicated GPU, with 256MB of memory dedicated GDDR2 or faster, with pixel shader 2.0 and full DirectX 10 support, Aero ran without "lifting a figure". In fact, on some laptops, using Aero SAVED battery life compared to Aero Basic (XP GUI engine, using mostly the CPU), as the dedicated GPU would still need to be powered at it's lowest power state/performance regularly Aero turned on or off. So if you didn't use Aero, your CPU was struggling and forced to go at faster speed.
Of course, if you didn't have the specs, then it was a nightmare, but that is beside the point.
Today, Intel integrated graphic solution is no problem for what Windows 7 and 8 GUI and in the case of Windows 8: Start Screen and modern UI app, though.
Ok so what does it have do do with anything, let alone blurry font? Font rendering, is surprisingly VERY complex and VERY hard, especially more so on the GPU. Any experts will confirm you this.
So, is it Windows 8 fault? No... it's the App in question. You have the similar blurry font effect, if you save a file as an XPS file. Microsoft own PDF alternative, which is fully GPU rendered as well.
Firefox and IE also faced the same problem. Firefox, after numerous tweaks, got it to look nice and sharp... IE... not so much, but it's ok. Still a lot of work.
So in the case of Adobe, it will take time before they polish their font rendering engine for their Modern UI app. Took Firefox a few years with a big focus on it, to get it right.