Questions about PDF in general (iText 7)

I want to know the difference or comparison between different PDF creation / generation techniques. For Example: iText, Adobe LC, Jasper Reports, etc.
I want to position text at some specific place in the document. However, I cannot figure out where to find the origin of the coordinate system of the page. What is the coordinate of the bottom left corner, top right corner, bottom right corner, top left corner? Where is this origin?
I am trying to find the page number of a PDF object using iText's Java API.
I would like to know more about the coordinates of a Rectangle, in particular: lower left X lower left Y upper right X upper right Y Every time, I get confused about how to make dimensions based on these coordinates to draw rectangle. If possible, can I get a graphical representation briefly about these coordinates positions?
I have the Java application that can create a PDF file. For example, I create a simple file from my program and I also wrote some code to open the file as soon as it's created correctly. However, if I want to run the code again, I must close the open file before I recreate it.
We need to encrypt a PDF with a certificate. I've found something using iText some months ago, but I cannot find it any more. The certs are on a smart card.
Can anyone please explain me what kind of differences exist between PDF files with the same content and explain why the PDF format has this defect if I may say.
I am serving a PDF to a browser via a Servlet. I need to disable the "save" and "print" option in the Adobe Reader menu bar while other options like "scroll", "find",... should be preserved.
Does a PDF file have styles, headers and footers information as is the case with docx files that have separate xml files with extra information?
We explored many API's like Tika, PdfBox and iText to extract page numbers from a PDF file, but we weren't able to meet this requirement. In iText we tried PdfPageLabels.getPageLabels(reader) but the behavior of this method is not uniform.