I securely store and work with pdf files. I like work with Pdffiller and I like many of its features, especially the left pane that shows the document pages, and the ability to select only those that you want to include in a print or save maneuver.
PDFFiller is our salvation. It is a software that doesn´t require any effort to understand because everything is as simple as upload the document, make the changes and save as pdf or print the document. I use it every day and I find it very safe and complete for the things that I need.