Personally, I think just storing all fields de-normalized in the invoice table (for generating reports), in addition to .pdf caching and an option to mass-download the .pdfs (for invoice retention), is the most practicable method.
Whatever method you choose, keep in mind that the invoices must be retained long term.
That doesn't necessarily mean they have to stay in Blesta long term, but if not, they must be easy to transfer over to your computer to store locally.
In that case viewing older invoices that have been transferred and removed from Blesta, must be disabled, and not allow the user to generate a new pdf.
By reading things like "Option to delete archived copy after X months" I think you may not have a clear idea about the actual legal non-negotiable requirements yet.
Most EU countries require around 7 years of retention for the actual invoices sent and received, see page 19 on: European Retention Periods.pdf
If this is a problem due to the size of Blesta's PDFs, than the first priority would be to reduce those, not embedding the unicode font in the pdf when no non-ASCII characters are used.
Other customer records (page 25) have a shorter retention period. E.g. you are supposed to delete the personal details of an inactive customer from your system within 2 years here (although that often does not happen in practice).
Storing customer data at Amazon AWS -or any other US company or hoster- like suggested in CORE-923 is problematic for EU companies.
First there are privacy laws that restrict the processing of personal data outside the EU. In short the company you are storing the data at must guarantee they comply with the EU rules (are on the EU-US safe harbour list), and you may have to inform your customers that you are shipping their data there...
Second some countries have laws requiring accounting data to be stored within their jurisdiction for enforcement reasons.
They do not like the idea of you being able to claim that you stored your accounting data on some vague cloud service in the other end of the world, and that it happens to be down, the moment the tax inspector pays a surprise visit...