I believe pdfs can load remote images, which pings a server. There are other reasons, I haven’t got sufficient knowledge. Some pdf readers will offer a sandboxed mode improving security. I think zathura has this for instance.

Tip: if you are going to store data on “the cloud” aka someone else’s computer, encrypt it locally before sending the data! There are existing solutions to do this in a cloud-optimized way (that is, not sending in an archive which has to be fully reuploaded for any tiny change) such as cryptomator. I think rclone also has an encryption feature but haven’t checked.

Goes to show how much of a joke copyright is. These laws are bought by and made to protect the interests of corporations. disgusting. rip aaron schwarz, your ideas live on.