If you use the user with ownership of the file, are you able to add the Administrator group, to the list of users with permissions to the file? Any additional screenshots will need an English translation, since all questions are expected to be in English instead of Spanish (which include screenshots).

Hi, yes, I'm able to add Administrators group to the list of users with pemissions to the file, I tried with 1 file, there was no problem, but I still cannot open the file. And sure, I will translate any additional screenshots.

You are adding permissions to individual files or are you adding permissions to the entire Documents folder? In either case, if you copy the file from the current location within Windows.old to a new location, are you able to open the file with the copy of the original file, doing this is a lot easier than fixing the permission so you can open the file in it's original location.

As an Administrator, you should immediately run DISM /Online /Set-OSUninstallWindow /Value:60 to avoid WIndows.old from being automatically purged. There is one solution I am hesitant to suggest, simply reverse the upgrade to Windows 10, then backup the personal documents. I assume no backups of these files exist?

@Lauro182 Please provide the output of icacls "C:\Windows.old\Usuarios\MARTIN CAMPOS\Documentos" within a code box in your question (please also use correct markdown for monospaced code, such as file paths, by enclosing them within single backticks - web translators intentionally don't translate monospace content since doing so makes the content illegible/wrong)

I have been doing this for the whole Documents folder inside Windows.old, but tried adding the Administrator group to 1 file to see if I could do it. I tried copying one of the files to another location, but it says I need permissions from current owner (which is the user signed in), but still it won't let me copy the file. Adding screenshot.

@JW0914 added output in edit

@Ramhound You are correct, there is no backup of these files, I was going to upload all these to a cloud before upgrading, but my dad got desperate and went on his own and got another guy to do this upgrade and this guy didn't back up anything... but yeah, I might trying rolling back if nothing else works, but I want to try everything I can, because I'm not sure if rolling back will work, or if it is even possible.

@Ramhound when should I run this command: DISM /Online /Set-OSUninstallWindow /Value:60?, purging this folder is the worst case escenario, I need to back up this file to another location, even if they aren't accesible at the time, but as you can see, I cannot even copy them, so I'm planning of cloning the disk before attempting anything else.

@Lauro182 - When you read my last comment it will prevent the folder from being deleted until that deadline has past. How long it's been I don't know, it's a precatation, incase it's been near the default amount of time

@Ramhound oh, ok, thank you, this was 2 days ago, will run the command ASAP, any more suggestion on what else I could do?

@Lauro182 I believe you pasted the wrong output for icacls, as it should look similar to this, showing the permission structure of C:\Windows.old\Users\MARTIN CAMPOS\Documents

@JW0914 oh yeah, my bad man, I misunderstood, I edited the answer to show proper output from running icacls "C:\Windows.old\Users\MARTIN CAMPOS\Documents"

@Lauro182 Try removing inheritance for permissions on C:\Windows.old\Users\MARTIN CAMPOS\Documents, converting them to explicit permissions - if that doesn't resolve, I would run ChkDsk C: /OfflineScanAndFix (I recommend booting to WinRE's command prompt, otherwise ChkDsk will run at the next boot, no progress will be shown, nor will any of the data from it.) Note: in WinRE, C: is usally not the OS partition - to ascertain: DiskPart→ lis vol → exit

@JW0914 He did not use the same username, should I just create another windows User with same Username as it was in Windows 7?

@Lauro182 It likely wouldn't resolve the issue since the SIDs would be different. This is abnormal and only asked in the off-chance something wonky occurred with the same username, albeit different SIDs. @harrymc's answer with running ChkDsk would be would I would do, as well as moving all files/subdirectories from within it via a Linux LiveUSB as harrymc recommended (Ubuntu install USB would work - choose to Try Ubuntu at boot)

Like the above user stated, is this an EFS formatted drive or NTFS ? Also, make sure to apply your changes to all subfolders (check "Reemplazar todas les entradas de permisos de objetos ....."). Alternatively, you can use a linux live CD to mount the partition (if its NTFS) and it will ignore the windows ACLs and you'll have full read access to everything.

@Silbee It should be NTFS, sadly, there is no backups of the disk, since the person who did the upgrade just did it over the same disk, all we have is the windows.old folder. I'm starting to get really desperate, it is over 20 years of work of my dad. I'll try the Linux thing as soon as I can, my dad will send me the computer tomorrow. I have not tried using the "Reemplazar todas las entrada de permisos de objetos ..." checkmark, I will try it later, I just to backup the disk even if the files are not accessible, want to make sure they don't get deleted by DISM Uninstall Window.

@w32sh I read the links provided, there seems to be another link on how to do it without the keys, but this have gotten way too technical for me, I'll try to keep studying on that, but after I try copying files with Linux. Thanks for the lead.