If you want to send a .pdf copy of your quote to your customers, you can turn this option on and off on each quote. This setting will set the "Attach PDF Copy" option to true on all new quotes. It's one less option your user needs to change on each quote ;-)
Under the Defaults Section:
If Attach as PDF is checked
All new quotes will have the Attach PDF Copy switched on by default.
Sending PDF copy of your quote
With the above setting checked all new quotes will have this toggle set to true by default. The emailed quote will send an exact copy of the quote in an attached .pdf.
This shows the toggle when set to true, and indicates that a PDF quote will be attached:
This is where the setting is located on the quote level.