Just print into PDF? It's a system function of any Mac.
I don't miss the feature "inside ProtonMail" (as a Mac user)

Of course, there is a workaround (but it's very painful):
1. Export E-Mail as ".eml" file.
2. Open ".eml" file with a freeware program e.g. EML Reader
3. Convert to PDF-File
4. Move PDF-File to external Archive-System
==> I find it a bit cumbersome. Don't you think so?

For archiving purposes it would be extremly useful (single e-mail to PDF-file),

I think this is not needed. You have on every OS a Print to PDF option (one way or another).

For the ones who ask for printing multiple emails to a single PDF:
1) well, just print your emails separately and use a free tool to merge your PDF files into a single one.
2) other email clients (Outlook), allow you to select multiple messages and print them bulk (again, you select as destination the "print to PDF/whatever you like as PDF" logical printer)

It already exist on the Mobile IOS app, not on the MacOS-app. I would love to have it on the Mac-OS version too!

Already exists! Although, a 'Print multiple emails to (single)) PDF' feature could be useful.

More specifically print/save multiple emails to PDF, which is important for legal issues and also Real Estate Transactions. Apple Mail does this very easily by selecting multiple emails, then Cmd/P and print or save to PDF, and it works great. However, it would be better to have this built into ProtonMail so using Proton Bridge just to save/print emails isn't necessary. Currently Proton Mail (MacOS) only allows printing/saving one email at a time, which is unusable for those in the Legal or Real Estate businesses.

Proton may as well host webinars on how to use a computer

Guided steps:

1. Print to PDF

Does this not already exist?
- open email
- click the dots next to the reply section (just looking on iOS at the minute but fairly sure I can do it everywhere)
- scroll down to “more”
- “save as pdf”

PDFs are buggy. The reason is the close source of the original engine. There is no reason to use old technologies as new ones are already here.

EPUB is a better alternative. Mails are htmls and are better to be stored the same way. Fonts can be stored as fonts and SVG vector graphics as... SVG.

I moved from mht and pdf to epub and happy with that.