For a client, I need to be able to read and write PDF files. There are two downsides to this:
The PDF format is actually standardized in the form of ISO 32000-1:2008. You can always purchase it from the website and implement a reader/writer/renderer yourself.
However, I'd strongly recommend against that, as the cost of the document and the cost in time of actually implementing something like this on your own will far outweigh the cost of a third-party component.
There are plenty of third-party components for .NET, some that (with iTextSharp being a very prominent one) are royalty-free as well.
I'd suggest convincing whomever set the requirement that you can't use third-party components to sign-off on the expenditure of one of these components (if none of the free ones do what you need).