I want to send mass emails with invoice pdf and ability to attached other files stored in the files subtab as schedule jobs. I can today send the email with the invoice as a pdf attachment but unfamilar how to attach other files (specifically uploaded to the transaction in the files subtab).. anyone has experience with this?

SuiteScript can achieve this, I believe.

any sample to share?

No sample. You will need to search the attached file and then attach them in your email.