Confiusing matrix, identificators and streams

Question

I've just signed a document using Acrobat Reader.

I've got that result:

Now I have these questions:

1. what does q, q means in the stream?

2. I have matrix [48, 0, 0, 17.5200043, 12, 27.2400055]. What does it mean, and how to calculate that? I'm going to create PDF by hand, and I'm interested in that. im0 is the name of Image (I have Visible signature in PDF).

3. Then I have Do Q Q. What does it mean?

4. Also, I have n0 XObject too. That stream is % DSBlank. Is it important? What is this for?

Answer 1

You need to start reading the PDF specification, you will not get far by asking questions here about each pdf operator.
1. The q operator saves the graphics state, the Q operator restores it. In your context 'q q' is redundant.
2. The matrix sets the image (/Im0 that follows) position and size. Elements 1 and 4 of the matrix are image size, elements 5 and 6 and image position (bottom left corner). This is a simplistic explanation, you should read the PDF specification to see how the trasnformation matrix works.
3. The Do operator paints the /Im0 XObject (your image).
4. % starts a comment line and % DSBlank is just a comment.

PS: The application you use for exploring the PDF structure is developed by the company I work for :)