Try this... create a blank invoice with Word (or whatever you want) and save it as a PDF.
Then use a PDF library to modify the PDF (insert the text at particular coordinates). We do this in the Microsoft world and it is extremely easy.
The biggest benefit is that we can use our own tools to create and modify the template. If we want to add some static text, we just crank open Word, make the change and save it to a PDF file (that is being used as a template).
For Microsoft, we use iTextSharp which is actually a C# port of the original Java version of iText
Additionally...
You can use Adobe Acrobat to insert fields in the PDF (address, phone, invoice number, line item 1, line item 2, etc...) and then use iText/iTextSharp to populate these fields at run time.
This is, in more detail, what we do... and it is extremely easy.