Combine multiple PDF files into one (arranged in a matrix)

Having a set of PDF files, lets say: in-01.pdf, in-02.pdf, in-03.pdf, ... I would like to combine all of them into a single one, forming an M x N matrix. The command montage allows for doing so (M and N should be integers): montage -mode concatenate -tile NxM in-*.pdf out.pdf The problem is the size of the resulting PDF is huge, while I would expect it to be (maybe just) a little bigger than the sum of all the input PDF sizes. I think montage is first converting the input PDFs to images and then creating the output PDF out of those images (so for example, the text in the original PDFs is not showed as text in the output PDF, but as an image with lower quality and bigger size). I guess there should be a way to do it (LATEX, for example, allows to insert a PDF image in another PDF without the need to convert it to an image first). I am looking for a command line alternative using free software tools under GNU/Linux systems. **NOTE**: we can asume those PDF files have all the same exact dimension (width and height). They are auto-generated PDF images normally consisting of a plot/graph (simple shapes line lines and rectangles) and a few text (title, labels...).