Security

Malware bytes 
Email phishing 
Sending files

Documents in PDF 

"If there’s a credible open-source equivalent to either of these options, we’d love to hear it."

Probably only Epesi (17 years old FOSS project) has this built-in out of the box function where you send a link to a file (e.g. PDF) that has its token and you know if a person downloads the file and from where, plus file version and access history… The process is as follow:
1. Task - e.g. report
2. All files used to generate the report must be included as attachments
3. Editable file like Word, Excel, Open Office, Pages - to be able to edit or recreate the document
4. Final PDF file to be distributed
5. If sent via e-mail then the link/token combination is used for security and access control. We don't send files directly as attachments. This way there is a reduced risk of attachments being too large, network traffic is reduced, files are not sent unnecessary to people who will never open them, smaller chance of email being classified as SPAM, reduced risk of virus infection etc.

https://hackaday.com/2023/06/15/the-simplest-social-engineering-hack-of-them-all/