Learn Italian: Gli azionisti non hanno affatto sottoscritto il documento. - The shareholders did not endorse the document at all.

In compound tenses like the passato prossimo, emphasizing adverbs like affatto (along with mai, già, and più) are typically placed right between the auxiliary verb (hanno) and the past participle (sottoscritto). While putting it at the very end of the sentence is sometimes heard in spoken Italian, this sandwiched position is the most natural and standard way to phrase it.

Could I use per niente instead of affatto in this sentence?

Yes, per niente is perfectly interchangeable with affatto here to mean 'not at all'. Because it functions the same way grammatically, you would place it in the exact same spot: Gli azionisti non hanno per niente sottoscritto il documento.

Sottoscritto looks like it should mean 'underwritten'. Why does the translation say 'endorse'?

You have spotted the literal translation! Sottoscrivere literally means to sign at the bottom (like a contract). By extension, it is frequently used figuratively to mean to endorse, to support, or to entirely agree with. Depending on the exact context, the shareholders here either refused to physically sign the document or completely refused to support its contents.

I know azione means 'action'. How does it relate to azionisti (shareholders)?