A PDF is hard to edit, and many upload forms and image tools want pictures instead. Markdown is plain text that stays readable and is easy to keep in version control. PDF is at home in documents that must look the same everywhere, and printing; MD is the usual choice for READMEs, notes and documentation.
The defaults suit most files. Under Options you can choose which pages to convert and set or supply a password.
Only the text and its outline (headings, paragraphs, lists) are rebuilt. Tables, columns set side by side and images do not survive, and a scanned PDF, which holds pictures of pages rather than text, yields little or nothing because ToAny does not do OCR. A password-protected PDF needs its password, entered under Options; without it the file is refused.