The most important tip is always to send in PDF, because Word can end up with weird formatting depending on how the person opens it, and PDF keeps everything the same. As for the email subject, be direct: "Application - [Your Name]" or "Candidate for [position] - [Your Name]" is enough. Nothing generic like "Hi" or overly explanatory. In the email body itself, you don't need a whole cover letter (like those giant texts), just write a short paragraph introducing yourself: your name, which position you're sending your resume for, why you're interested in it (something brief, like "I have experience in X and I think it's relevant for the position"). Then attach the resume and you're done.
About formatting, send the PDF with a descriptive name like "Resume_YourName.pdf" instead of leaving it as that generic "document1.pdf". Some companies have those portals where you fill everything in directly, so you don't even need to worry about email, but when it is through email, these tips work well. The thing is to be clear and professional, but without sounding like a robot. Like, you're delivering a product (yourself), so present it all nicely wrapped up haha. Good luck with your resumes!