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).
Agree with the answers above. Implementing a PDF engine isn't a trivial task. If you need to do the full one reading and modifying PDF, then use third party library like suggested.
I have worked with PDF document creation, management and etc for several years. Often I found that the requirement to handle PDF can be very basic and achieved quite easily but misunderstood. I will just list some options here to consider if one of your requirement fall in to this category:
As other answers stated writing your own PDF handler will be a pain. You can try using PDFsharp which seems promising and is free. You have to check the license of usage thou.