[phpdave11]

It's really not that difficult if you read and understand the PDF specification. As a learning exercise, I created a simple PDF generator library that creates ASCII PDF documents (you can open them in Notepad) and includes comments about what each drawing instruction does.

https://github.com/phpdave11/davepdf

[recursive]

I'm sure generating PDFs is much easier than reading them, such that "it just works" with any kind of PDF.

[phpdave11]

Reading them is also easy. I wrote a library that reads PDFs and imports page(s) from an existing PDF into a new PDF as a Form XObject.

https://github.com/phpdave11/gofpdi