'Export as pdf' is not one of the  'Export as' options on  a Mac. Instead, you should follow these instructions, from

?pdf:

 
1. Open the worksheet that you want to export.
2. From the File menu, select Print.
3. Specify options for export to PDF as described in Section Options for Export to PDF below.
4. From the PDF drop-down list, select Save as PDF...
5. In the Save As dialog, enter a filename.
Click Save.