I got a email with the following requirement,

I came across your post while searching for a solution to add barcode to my invoices.
I can print the invoices as a pdf file but the problem is to have my tracking number printed as a barcode.
Can you do some software customization so it can read a certain [...]

I tried to install Microsoft XML Paper Specification Essentials Pack(aka XPSEP or XPS Essentials Pack) many times on my PC, but failed, after searching, I got the answer. Then I thought how about just copy the necessary file on a fresh PC to run the XPSEP, after tested, it works. So I share it [...]

If you want a c version pdf tool kit, similar with pdftk, I think you can consider of mbtPdfAsm.
The features,

Encrypt PDF with standard 40 bits or 128 bits, if you want decrypt function or AES-128 support, please try pdfcrypt.
Rotate all the pages of the destination file. Value is 90, 180 or 270. My pdfrotate [...]