Inkscape.
Some people mentioned Inkscape and I can't recommend it enough because it's my goto FOSS PDF editor.
It's not made for PDFs and it shows, bit regardless it's absolutely incredible how versatile it is.
You can keep the formatting, it's vector so no loss of percieved quality, and its Text tool is easy (and fast) to use.
The only problem is each page has to be imported and exported seperstely - you'll have to use something else to combine them
As far as signing goes, if it can be a classic squiggle it's perfect - there's a few pen tools and one has smoothing so you can play with it a bit until the signature looks good.
On mobile, so excuse any typos.