Hi David,
There is a lot more work to writing a PDF reader.
However, if you've already spent a few thousands hours working on creating PDF output, writing a PDF Reader seems much less daunting. <g> Because there are a lot of things you probably don't want or have to support.
But, by supporting all of the available PDF Features in Adobe PDF for every single PDF document that you load regardless of whether or not it has those features allows you more time to run and get a snack while you wait for the Adobe PDF Reader to load your document. <g>
That's why writing a PDF output creator is much easier than writing a PDF reader/viewer, since the reader/viewer must deal with features of many versions while for PDF output you can in most cases use a simpler set of features.