As a supplementary answer to Fattie's answer, regarding "future unemployability" (and in part addressing this comment):
Where would this be recorded and shared with people who would employ you?
in case there is any doubt as to whether news of your sharing a confidential letter either as a whole or in part (i.e. redacted) would get out... bored people in offices/workplaces talk... a lot!
First, around your immediate locale/site
So, just from standing around the water cooler, it is likely after a while you would be known for having shared a confidential letter from a prospective employee, and even the contents of that letter may become common knowledge:
Hey, did you hear that person X got offered 10 Escudos more per hour by company Y?
Second, beyond your workplace
Then, take into account, that news can casually spread further than the immediate locale/site because friends/colleagues/acquaintances across sites talk - for example, in the pub at lunch, when entertaining cross-site (including multi-national) staff members, external third parties, and other visitors. People often see little harm in sharing details like these, with others who aren't immediately associated with the person concerned. Sharing details like these can be seen to help create an intimacy that build external working relationships, and may even help close a deal.
By now, your sharing of a confidential letter has become common casual knowledge, even though it still seems like harmless banter/gossip, and may not seem all that problematic.
Third, at job application time
The problem comes when you apply for a related job elsewhere and the immediate hiring team - not the HR team but the technical team members who are conducting the technical aspects of the interview (the people that you will end up working with and who (usually) have a major vote in your eventual hiring) - contact their friends from other sites and companies (that could be miles away or even in a different country) and ask
Hey, you worked with, or closely with, or with a team that included person X. What are they like?
To which the response could be one of these three:
- Awful, avoid them like the plague
- Amazing, hire them immediately
- Ok, but let me tell you a story about them sharing a confidential letter...
You think this is unlikely? Trust me, this happens a lot.
Now, indeed, this may not make you unemployable, but, regardless of whether it does or does not, do you want to be known as the person in this story?