Recovering “Damaged” Files on External USB Drive: GUID, HFS (can not open files)

I had some files on an old thumb-drive, and recently I was going through and discovered that all/most of the files were no longer readable by any of my macs. I seam to remember having similar problems like this while back, and finding an apparent link to my choice of format, so I ran diskutil list in terminal:

/dev/disk3 (external, physical):
   #:                       TYPE NAME                    SIZE       IDENTIFIER
   0:      GUID_partition_scheme                        *134.2 GB   disk3
   1:                        EFI EFI                     209.7 MB   disk3s1
   2:                  Apple_HFS my-old-files            133.9 GB   disk3s2

And I tried ls -l

total 3858320
-rw-r--r--@  1 kipbits  staff     3430100 Nov 21  2016 2015-12-24.pdf
drwxr-xr-x   8 kipbits  staff         272 Sep 26  2016 2016-sep-14-files
-rw-r--r--   1 kipbits  staff    25642888 May 18  2015 myold.mp4
-rw-r--r--@  1 kipbits  staff         884 Jul 31 20:20 someold.txt
-rw-r--r--@  1 kipbits  staff  1320157184 Sep 21  2016 someold.iso
drwxr-xr-x   4 kipbits  staff         136 Sep 27 16:23 anotherdir
-rw-r--r--@  1 kipbits  staff   626214959 Nov  5  2016 someold.mp4
drwxr-xr-x  14 kipbits  staff         476 Nov  2  2016 sept2016files

I think this looks normal?

The problem is, I'm not able to open any of my files. I get the following errors:

pdf:

**The file “some-old.pdf” could not be opened.**
It may be damaged or use a file format that Preview doesn’t recognize.

txt:
800 byte files are blank

rtf:

**The document “some-old.rtf” could not be opened.** 

mp4:

**QuickTime Player can't open "some-old.mp4".**
To see if additional software is available that will enable QuickTime Player to open the movie, click Tell Me More.

dmg, iso:
will mount but did not test further

pkg:

**The operation couldn’t be completed. (com.apple.installer.pagecontroller error -1.)**
Couldn't open "some-old.pkg"

I found a thread on apple saying "The partition map type doesn't really make any difference for non bootable volumes", although i don't get any of these problems when using Master Boot Record & ExFAT.

None of the files are more than a couple years old, so i don't think there should be any compatibility issues, and none of the files were corrupted when I copied them to the drive. I have had this same problem with other drives, and they were perfectly usable after reformatting, although this involved erasing files.

My question is: Is there a way to recover my files? The file-sizes still display that the files are there. Is there a way to save these files and make them readable?

…Also, if it makes any difference, I am using El Capitan(for dev purposes)

Update————————————————————————————————(04/Oct/2017)

I did a hexdump for some of the files in question. my output was something like this:

 0000000 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
*
 0000370 00 00 00 00                                    
 0000374

what does this mean? do you think a recovery app could even help me here?

here are some threads that are perhaps related:

…Also could this possibly be related to SIP? I first noticed this problem around when i first upgraded to El Capitan.

Or perhaps there is something else going on? I have recently been experimenting with windows and various forms of linux, and have been moving files around with thumb drives. Perhaps I plugged one in to the wrong computer and corrupted all the files?

Whatever the cause, I'm wondering if the files might still be recoverable? Is hexdump a good way to diagnose recoverability? Would buying a recover app just be a waste of $100?