PdfBooklet is a Python Gtk application which allows to make books or booklets from existing pdf files. It can also adjust margins, rotate, scale, merge files or extract pages.
PDF Combiner is a user-friendly, GUI-based tool built in
PDF Combiner is a user-friendly open source free to use, GUI-based tool for combining, pdf to excel, pdf to word, image to pdf, zip, unzip annotate and splitting PDF files. It is easy to use, supports multiple file insert and delete and process, and allows you to adjust the order of files before combining.
...Unfortunately not all PDF viewers know how to deal with attachments. PDF viewers known to support downloading of file attachments are: Acrobat Reader, pdf.js and evince. The pdftk CLI program can also extract attached files from a PDF. Preview for OSX does not know how to display/give you access to attachments of PDF files.