1. Depends on the size of your pocket. It's a 5" device with minimal extra area outside the screen, compared to the 6" Kindle with keyboard. It fits into the front or rear pockets of my Levis (it's almost exactly the size of the rear pocket). I don't know if I'd call it comfortable, but it certainly fits.
2. Not sure. I wasn't impressed with its PDF reading capability. But then I don't believe PDF is a valid ebook format, so I don't use it. If something is
only available in PDF, I do my best to convert it to something more readable. I realize this isn't always possible for technical material, but I don't use my reader for that.
3. Depends on the format and DRM. For example, if they use PDF or EPUB with Adobe's ADEPT DRM, the device will read them. Note, however, that there's a long-standing bug when you have different DRMed formats on the device at the same time.
4. Get
Calibre. It will do everything you need except register for DRM. On a Windows machine you need Adobe Digital Editions or the Sony app for that. I'm not sure what you'd do on Linux. But Calibre will happily sync files for you.
5. I'm not quite sure what you're referring to. The Sony has 3 levels of text sizes, if that's what you mean. It can also do some zooming on PDFs. Otherwise, the answer is probably "no"?
6. I've not used a Kindle, so I can't give a comparison other than that they both use the exact same e-ink technology, and at 5" the Sony screen has a higher pixel-per-inch density.
For comparison, my other device (and main reader) is an iPhone and I mostly read linear fiction/non-fiction. I really like my iPhone, and only picked up the Sony on a whim because I got a good deal at Woot. For me, having a backlight is a major requirement, and I find reading on the LCD screen of my iPhone comfortable enough. I realize others are the opposite -- they can't comfortable read on an LCD and don't mind the lack of backlight on e-ink devices.