I agree, the PDF spec is great, and very easy to understand (if slow to wade through). The hardest parts are when you have to duck out to read another spec for a contained format like TrueType.
Regarding Reader, I work with PDFs a lot, and the majority of issues have a fairly common pattern. The supplier has created a PDF in a 3rd party tool, which is invalid in a subtle way (production printers in particular are very specific about what they want to accept).
But it works fine in Adobe Reader, since it was built to be very tolerant in what it accepts, so it's often hard to convince the non-technical users that the file has an issue. It's great for end users but has meant that a lot of tools out there just didn't have to try too hard to make PDFs that mostly work, so programming workflows can be an issue.
Regarding Reader, I work with PDFs a lot, and the majority of issues have a fairly common pattern. The supplier has created a PDF in a 3rd party tool, which is invalid in a subtle way (production printers in particular are very specific about what they want to accept).
But it works fine in Adobe Reader, since it was built to be very tolerant in what it accepts, so it's often hard to convince the non-technical users that the file has an issue. It's great for end users but has meant that a lot of tools out there just didn't have to try too hard to make PDFs that mostly work, so programming workflows can be an issue.