Use uppercase and no period in:

• buttons
• checkboxes
• radio buttons
• menu items

✗

.PDF, .CSS, .HTML

✓

PDF, CSS, HTML

Use lowercase and specify a noun after the extension in full sentences in:

• tooltips
• notices
• messages

✗

Add a DOCX file. Save your file in the pdf file format.

✓

Add a .docx file. Save your file in the .pdf file format.