Our current invoices sometimes have up to 4,000 lines which we send to customers as a CSV attachment to their PDF Invoice. Currently the Invoice PDF is also 4,000 lines. We've created a summarized saved search which can display what is needed in ten lines (but we sill need the actual 4000 lines for other processing within NS transactions.) Can we load this saved search in the Invoices Advanced PDF and replace the default record.items with our reduced lines? Or can we do the summarizing in freemarker?

Definitely, you can do it in freemarker, depending on your requirements and invoice setup. I think this https://suiteanswers.custhelp.com/app/answers/detail/a_id/77755/loc/en_US and/or this https://suiteanswers.custhelp.com/app/answers/detail/a_id/33495/loc/en_US should get you started on that path. TBH, I haven't done this, but I guess you could also probably do it via a template for your saved search (this seems likes a good place to start: https://suiteanswers.custhelp.com/app/answers/detail/a_id/63077/loc/en_US) Feels like it'd be easier just to modify the invoice template, personally, versus having to create the whole invoice inside a saved search form, but as always, the devil is in the details.

The article "Group Printed Items on Advanced PDF/HTML Templates" looks like a good inspiration for what we want.