You want to fill in PDF forms on Ubuntu : here is a solution.
You want to modify PDF on Ubuntu : here is a solution.
You want to merge PDF files into one and/or split one PDF file into many: here are your solutions.
You want to convert PDF files to JPEG images, here are the solutions with Ubuntu !
Exceptionally, a post about Windows: how to create PDF files easily ... and legally.
Of course, Linux does that out of the box ... but that's another story