How can I combine PDFs in LibreOffice?
To combine pages from two PDFs in LibreOffice Writer follow these steps.
Open the first PDF document in LibreOffice Writer using File -> Open.
Open the second PDF document in a new LibreOffice Writer window using File -> Open.
Select a page in the first PDF document Pages pane before where you want to add a new page in the PDF document.
Select the page you want to add from the second LibreOffice window and copy it.
Paste the new page into the first LibreOffice Writer window.
Export the new PDF file using File -> Export as PDF... -> click the Export button -> click the Save button -> browse to the location where the newly created PDF file will be saved.
LibreOffice Draw
It seems that you can combine them in Draw.
http://www.techrepublic.com/blog/tr-dojo/edit-pdf-documents-with-libreoffice-draw/
pdfsam
However my real suggestion would be to use
http://www.pdfsam.org/
GhostScript/PDFTK
Edit: On Linux you can use GhostScript and pdftk
https://stackoverflow.com/a/8159842/2982543
http://www.ghostscript.com/download/gsdnld.html
https://www.pdflabs.com/tools/pdftk-server/#download