I use a program called CamScanner for my Galaxy S5 phone. I use it specifically to scan documents which I need to keep as a backup and it makes my world a lot easier to do so by having a digital copy of it rather than a paper copy. I do this for 99.99% of my receipts (tax time) as well as important documents which I require a digital copy of right away but cannot find a hardware-desktop scanner which will give me a PDF file (they do exist btw) but they do not work in the same manner. I started using CamScanner because it was ultra fast to do and I can scan, make a PDF or JPG of the image right away, email it to someone or to myself or simply have a digital copy whenever I need it right on my phone.
I recently factory reset my GS5 to factory settings as it was getting pretty slow, I backed up the hundreds of PDF's I've made over the years (have been using CamScanner for well over 1.5 years since I got the GS5) and when I reinstalled the purchased android app it started repopulating all my files. Which is pretty good for a one time fee. If you make an account on their website you can access your files through their them. How secure are they? Well it allows you to password lock your PDF's, similar to Adobe's PDF security.
As always, if someone is able to get into your phone they will be able to open the app itself but if there are certain documents password protected they won't be able to see them. While the level of actual security is questionable on the app server side, you probably should not keep anything super sensitive in the program and I do not know the level to which they keep backups of the documents you scan, or if they do so once you delete them from the program. For the regular use to which people may use such a program it may be most beneficial for those who need an instant PDF of a document. Try it free before you buy it, but it has serious limitations. I ended up paying the $3.95 (its $1.99 now) and it has become one of the more used apps on my phone.