There are two ways to convert entire body in pdf.
1. Share the email with Google keep using in built sharing menu, entire body along with headers will be copied to keep. Then share keep with Google Drive, and save as pdf.
Method 2
Through inbuilt share menu, go to "copy to clip board", copy the text in say Polaris office, and select, export to pdf, if you copy to say kingsoffice, then select print command and then print as pdf. Posting images