While preparing such a presentation - like now - I often come at the point, where I have to combine "god-given" pdfs, i. e. without having the sources, into one file. It's easy using ghostscript:
gs -sDEVICE=pdfwrite -sOutputFile=newfile.pdf -f file1.pdf file2.pdf file3.pdf
For instance, this is a good resource for pdf-related ghostscript commands. It also shows some examples, how to rotate pdf pages etc. See your favorite distribution for more details about ghostscript.