PDFs in Dropbox, stored in an encrypted sparse disk image (https://www.sas.upenn.edu/computing/natsci/sparse https://www.sas.upenn.edu/computing/natsci/sparse).

PDFs in Dropbox, stored in an encrypted sparse disk image (https://www.sas.upenn.edu/computing/natsci/sparse https://www.sas.upenn.edu/computing/natsci/sparse). I keep an additional copy of the encrypted disk image on a flash drive in one of those fire-resistant lockbox things.