AlivePDF is an open-source ActionScript 3 (Flash, Flex, AIR) PDF generation library. It allows you to generate PDF‘s on the client-side.

You can use AlivePDF with Flash, Flex and Adobe AIR applications. Some sample codes can be found here.

There also few tutorials about using it with AIR and Flex here. 

If you need a powerful PDF library for ASP.NET, you may find many paid solutions & it is hard to find a free one.

PDFsharp is an open source PDF library for ASP.NET that makes it possible & easy to create PDF files from any .NET language.

Using PDFsharp you can:

• create new PDF documents with text, lines, curves, images, etc.
• modify, merge, and split existing PDF files or incorporate pages from existing PDF files into new PDF documents.

PDFsharp, which is written completely in C#, includes MigraDoc lite (in German) which gives you more functionality like dealing with a document consisting of sections, paragraphs, tables, charts.