For saving PST files into PDF you have main two ways manual and automatic let's see both one by one:-

Manual Process

Step 1: Open Outlook: Launch the application and select the email you wish to convert.
Step 2:Access Print Menu: Go to the File menu and select Print.
Step 3: Select Printer: Choose Microsoft Print to PDF from the printer options.
Step 4: Save as PDF: Click Print, then choose a location to save the file, name it, and click Save.

Automatic process

Use a WebbyAcad PST Converter Tool.

Step 1: Install the tool from Webbyacad's official website.
Step 2: Open the software and Add your PST File by Add a File Button.
Step 3: Watch the file preview.
Step 4: Select the File Format PDF.
Step 4: Choose the saving location and Enter the Export Button.

For more further deatils:- https://www.webbyacad.net/pst-converter.html