The only cnv to pdf conversion that makes sense is an export or printing of Canvas drawing (.cnv) as a PDF document. This can be done in Canvas using the default export function or through the print menu.
Note: Many sites also claim that they can convert Microsoft Word .cnv files to PDF, but the conversion is simply not possible. This type .cnv file is similar to a .dll library and thus it contains no actual data from Word. You can only convert actual documents (.doc, .docx etc.).