Get a service like LastPass. It's primarily a password manager, but also has an encrypted vault to store sensitive files on their servers. I wouldn't trust any third party app that claims to make secure/private folders on your phone. Too often those lack even basic security and are easily bypassed. For example, they may simply rename a given file with a bogus extension in the name, leaving the file itself untouched and readable by the correct app (a pdf reader in your case). Or they could move the files to a new folder with a period in front of the folder name to hide it from general browsing, but still easily found using the "show hidden folders" option in a file browser.
When it comes to privacy and security, if it's free, it's likely worth every penny.