Saving data, in this case emails, as PDF is not a function that is limited to Outlook. You can do the same conversion in Word. For example, you can also convert Excel files to PDF.
You can open PDF files with Adobe Acrobat Reader or an appropriate browser plugin. Often saved PDFs are read-only. If you want to edit PDF files, you can get some helpful information by clicking through to the article.
When you convert emails to PDF, depending on the size of the email, the resulting file sizes may be quite large. It’s a good idea to shrink PDFs. Using the information listed in the link, you can lower the size of your file and send recipients compressed PDF files. Big attachments tend to clutter up inboxes.
If you tend to save sensitive information as PDFs and want to protect them, you should encrypt your PDF files. In this way, your documents remain protected from unauthorised access and cannot be opened without the appropriate credentials.