How do I place several pdf files into one new pdf file with only one (large) page?

Q: I’ll like to nest several pdf-files into one new pdf file with only
one (large) page.

I’ll like to place, scale and rotate (Matrix2D) each of the input-pfd-
files.

In your sample, you show how to merge PDF files into different pages.
That not my issue. You have a sample of placing several image files on
one page, but I cannot figure how to place a new pfd file.
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A: You can tweak ImpositionTest sample project (http://www.pdftron.com/
pdfnet/samplecode.html#ImpositionTest) so that all pages from an input
PDF document are placed on a single PDF page.

To implement this you would move the following code outside of the
loop - i.e. just before the 'for' loop.

  Page new_page = new_doc.PageCreate(media_box);
  new_doc.PagePushBack(new_page)
  writer.Begin(new_page);

You would also need to set the transform on each Form XObject element
so that it first into the corresponding cell of the master page. You
may need to come up with number of columns and row on the target page.
The overall pseudo code may look as follows:

Page new_page = new_doc.PageCreate(media_box);
new_doc.PagePushBack(new_page)
writer.Begin(new_page);

double cell_width = new_page.GetPageWidth()/cols, cell_height =
new_page.GetPageHeight()/rows;
for (int y=0; y<rows; ++y)
  for (int x=0; x<cols; ++x) {
  Page src_page = (Page)imported_pages[x*y+x];
  Element element = builder.CreateForm(src_page);
  double sc_x = cel_width / src_page.GetPageWidth();
  double sc_y = cel_height / src_page.GetPageHeight();
  double scale = Math.Min(sc_x, sc_y);
  element.GetGState().SetTransform(scale, 0, 0, scale, cell_width*x,
cell_width*y);
  writer.WritePlacedElement(element);
}
writer.End();

If you are dealing with several separate PDF documents, you could
import them into a single 'imported_pages' Array (i.e. call
new_doc.ImportPages() multiple times) before going on the place all
pages on the master page.

Q: I knew the procedure of setting the media box.

Yes, I have one single imposed page.

I want to create a booklet from some pieces of the single imposed
page.
I want to crop some regions of the page and paste them into a new
booklet
(every region is a new page), but not with setmediabox method.
This because the setmediabox doesn't really crop the page, but only
moves
the mediabox.

The final size of the booklet pdf will be: single_imposed_page_size x
number_of_pieces.
I want to really crop a portion of the page.
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A: You can use PDFNet SDK to remove page content that falls outside of
given bounding box. The code would look very similar to ElementEdit
(http://www.pdftron.com/pdfnet/samplecode.html#ElementEdit) sample
project but instead of skipping images during element copy, you would
skip any element whose bounding box does not intersect the new
clipping box (element.GetBBox(bbox); if bbox.IntersectRect(bbox, clip)
==false -> skip).

Having said this it is probably still more efficient (and less error
prone) to create a single form XObject (using
element_builder.CreateForm(page, doc) - similar to ImpositionTest
sample) and then use the same object on different PDF pages (with
different transforms - element.GetGState().SetTransform(scalex, 0, 0,
scaley, offsetx, offsety)). Please note that because the form xobject
would be shared between all pages the resulting file size would be
roughly the same as the original PDF.