Welcome to my nightmare, the worst research possibile: tools to modify pdf for free.
As simply, as COMPLICATED, since many tools are not safe (they use your data), or incomplete, or they require expensive subscriptions and so on.
I will share with you my solutions, which I know it is very fragmented but at least it appears to me to be the safest and the most affordable.

PREVIEW (only mac users)
Preview (preinstalled in OS system) is wonderful. Fast, easy, light – delete, move, rearrange pages, add notes and text, add your signature. FOR FREE.
PDF-XCHANGE EDITOR
pdf-xchange editor – as preview: delete, move, rearrange pages, add notes and text, add your signature, for free (for now).
ADOBE ONLINE
the official Adobe page (ONLINE) – delete, move, rearrange. Guaranteed by the official and professional app of Adobe
VIRTUAL PRINTER FOR CAD
pdfarchitect – it works with nanocad
SAVING IN PDF/A
pdfcreator – or online tested links: from pdf to pdf-a
Please feel free to add your suggestions.