Problem opening pdf files on macbook pro

I am having trouble opening some of the .pdf files in our Dropbox account on my Macbook Pro. Background: My techs edit pdf files out in the field on ipads and save them in a folder called UNSUBMITTED in our Dropbox account. In addition, some of the techs who don't have ipads, fill out a paper copy of the files and give it to me to scan into and save in the same Dropbox account. I scan the hard copies using an Epson scanner connected to my macbook pro from home. All the files (whether they were saved by the techs on their ipads or scanned in by me) will open fine on the office computer which is a windows computer. Some of these same files will not open up on my Macbook though (I get the error "file could not be opened. It may be damaged or use a file format that Preview doesn't recognize."). And it's not consistent....sometimes files that I scanned have the issue and sometimes the files that the techs have saved themselves have the issue. One of these threads said to Open Info on file and click on Open With - mine says Preview and there is nothing I can change that to. Could something be wrong with whatever software is on my Macbook which opens .pdf files? There is no rhyme or reason why some files work and some don't. The only constant is that all files can be opened up on the windows computer in the office. But they can't all be opened up on my Macbook. Help!