TechRepublic had a Michael Kassner article online April 15th (2013) entitled "DropSmack: Using Dropbox to steal files and deliver malware." The point was that Dropbox was not safe as files could be compromised.
A follow-on article by the same author on April 29th, "BoxCryptor vs. Dropsmack: The battle to secure Dropbox" stated that Dropbox files CAN be encrypted before loading (and getting Dropbox's security) to protect the files.
PM me if you want PDFs of those articles - they were worth saving!
The pair of articles provided a valuable reminder that the "soft encryption" of common programs are not sufficient to protect valuable information.
For nearly 20 years, PGP (Pretty Good Privacy) was the defacto encryption program for the general population. NSA tried to crack it and then attempted to have it made illegal but only succeeded in making it available within the US - not for export - so an international version (identical functionality) was made available, too. PGP was sold to a security firm which took the product commercial circa 2000 so it's lost a lot of its following ... but it remains an excellent encryption program.
Finally, Security is the proverbial three edged sword as there must be a tradeoff between cost, convenience and the level of protection to be given the data to be protected. If you want military grade encryption, it'll cost big time!
Back to the original question, e-mail can easily be captured so it is not safe. Dropbox has been shown not to be safe without pre-encryption. PGP has been around for ages and is still quite strong (but inconvenient) so other programs like BoxCryptor and TrueCrypt are finding a niche which you can use them to protect your files.
Finally, you are spot on that Acrobat passwords are trivial to break. There are programs out there which even allow "script kiddies" to do it, too. As for edit/print protection passwords, they are bypassed by simply saving to another format.