Unless a company specifies a specific format, you should generally send your cover letter as a PDF. That way, it’ll look exactly as you want it to, no matter what formatting or fonts you have in it, and it’ll look the same both on the web and on your phone.
You can print most documents to PDF on a Mac by going to print the document, then clicking “Save as PDF” at the bottom of the print dialog:
You should definitely save it as a PDF as that’s a universal format and the way you save it is the way the employer will see it. If you save it in word, the employer may have a different version and that can cause formatting issues.
Make sure to not only send your cover letter in PDF format, but your resume too! It's super important to stay consistent across documents and file types.
PDF is usually better, and will look professional, but be aware that the computer systems used by some companies to manage job applications extract the text from cover letters (and resumes). Fancy layouts can break these systems, and in some cases your application could be completely ignored! So keep it simple.
PDF is the way to go
Definitely do a PDF. PDFs are compatible with most systems so what you see is exactly what they will see. The formatting won't be affected or rearranged, which tends to happen when Word files are opened on some systems.