How do I turn an email into PDF format?

Assuming that you are using the native Mail App.

With the email that you wish to convert to a PDF is on-screen, tap the arrow icon at bottom-right of the screen; an action menu will appear. From the menu, select Print; the Print page will now appear, with the email paginated as thumbnail images on the left - and print options to the right…

Use the two finger Zoom gesture (i.e., spreading two fingers) on any of the thumbnail images; this will open a print preview page. At top-right of the print-preview, tap the Share icon (a square, with an upward-pointing arrow).

Tapping the Share icon will expose the Share Sheet; you’ll note that the document type is now a PDF as illustrated. You can now use any of the available Share actions - such as Save to Files.

Having saved or otherwise shared the PDF, just tap Done at top-left - and then cancel the Print dialogue.

This article might help you:--> https://appletoolbox.com/save-email-as-pdf-iphone-ipad/

or, use a 3rd parties apps (i.e. PDF Expert by Readdle).

Regards

Giulio