If the file is not already in PDF format, you can use Attach Plus to convert it to a PDF "on-the-fly". Attach Plus comes with a built-in PDF print driver for this purpose.
You can use Attach Plus to convert a file to PDF in two ways:
Use the Attach Plus print driver
Select File > Print from your application's main menu
Select either "Attach Plus - Save and E-mail PDF" or "Attach Plus - E-mail PDF" from the list of printers, and click Print
This will convert the file to PDF, then bring up the Attach Plus dialog to attach (the "Save and E-mail PDF" print driver will also prompt you to save the PDF copy)
Using the main Attach Plus "attach" dialog
Select the PDF file format from the Attach Plus dialog
This will convert it to PDF as part of the attachment process
Note: If your file is already in PDF format, you will not see the "PDF" option displayed.