Questions about PDF in general (iText 5)

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I'm using iText to sign and timestamp PDF documents. It works very well. But I recently switched from Acrobat Pro X to XI and now I see this new line: the signature is not LTV enabled and will expire after Now I'm a little bit confused: since the signature was declared valid at a known and certified date, why would it become invalid in the future?
How do I make a PDF document LTV enabled without using timestamps? I tried signing a document with a digital certificate in acrobat and when opened it says the document is LTV enabled. I did not use any timestamp. How can I do this using iText?
I use PdfStamper , but I don't change anything. I just take the original metadata and I put it back unchanged. When I calculate a hash for both files, I get 2 different hash results. May I know why?
Let's say I have a private teaching forum, where each user has a username and an encrypted password (paid account), and there's a DOWNLOAD section where pdf files can be downloaded by users only. These files should be protected by the personal user name and password of these users.
How to set password for an existing PDF?
I need to rotate a PDF page by an arbitrary angle, but it seems that the functionality to rotate pages is restricted to multiples of 90 degrees. The contents of the page are vector and text and I need to be able to zoom in on the contents later, which means that I cannot convert the page to an image because of the loss of resolution.
Some days ago I started to play with the internal structure of PDF document. While searching the internet, I've found some nice tools to edit the metadata, so far I haven't found how (and if) I can edit the page count in a way it won't affect the way the PDF is visualized. Can I change some metadata field, so that I can see a "fake" page count? If so, how is it done? Where can I read more about the internal structure of a PDF document?
We are using iText to decompress PDFs, but before doing so, we want to know if an existing PDF is already compressed or not. Is there any way we can check if a PDF is compressed or not?
am generating a PDF document and displaying it in a Web browser (the current version of IE is the most important target). I want to suppress the floating toolbar that appears and disappears depending on mouse movement.
I am using iTextSharp in a web application to generate PDF files. These PDF files contain .tiff images taken from a folder. If size of the contents of such a folder goes beyond 1GB then the browser gets closed automatically when receiving the PDF file. Is there a size limit when generating PDF files?