pdfseparate
and
pdfunite
commands provided by the poppler-utils .
package can modify PDFs directly from your Terminal. All These tools allow you to remove, add, and rearrange pages and export it to a new document.
These tools are also useful when creating PDFs with Inkscape. Inkscape currently only supports single-page export of PDFs, so you can use these tools in your workflow to join PDFs together after using Inkscape. Inkscape also supports importing a single pages of PDFs, so if you need to edit the actual content of a PDF Document, give Inkscape a try.