Andy-get Office for Mac. Bit arse about tit using MS software on Apple hardware but it works for me.
Ste-create an invoice in word. Press CMD & P together.
A box comes up bottom left hand side says PDF with a drop down arrow.
Click that. Choose 'save as PDF'
Create a name and choose a place to save it.
Perhaps 'Invoice Mrs Smith'
Save to desktop.
Then when you exit word, click 'yes' to save changes.
Then all you have to do next time the invoice is due is change the date.
Email over your PDF or print it, send to customer.
Create 3 folders.
Invoice Templates-word docs that you can change the date on each month.
Invoices paid-PDF's
Invoices outstanding-PDF's
Put all newly created invoices in the 'outstanding' folder.
When they are paid, drag & drop to the 'paid' folder.
Simples.
Perhaps there are better ways of utilising MAC's capabilities but its worked for me since getting my MBP.