I have the old version of goodreader for ipad IOS 5 When i have updated my ipad to IOS12, the goodraeder didn't work, as it is not compatible with the new IOS. How can i restore my PDF files from goodreader.

You would have needed to backup your iPad and those files to either or all Apple iCloud, DropBox, Google Drive/Docs or to a full computer, running Apple iTunes to back up all of your iPad info before you upgraded that iPad to iOS 12.5.4.

As it stands now, it maybe too late and those files maybe gone forever.

Newer versions of GoodReader had an automatic/import ability to transfer older, 32 bit files to the newer 64 bit app.

Not sure if this functionality is long gone, or not, as GoodReader developers had put this feature in place many years, ago, to help its user base migrate all older, 32 bit GoodReader app data to the newer 64-bit versions during the 32 to 64-bit code transition from iOS 10 to iOS 11 ( which was the first version of the 64-bit iOS coded version ).

The ONLY way to test, is to download the newest version of GoodReader and see when it first launches, if it puts up a pop-up message that states something to the effect of would you like to migrate your old GoodReader files to the newer version.

If this does NOT happen, then there is nothing more to be done.

iOS is now iPadOS and you and this iPad are almost 3 versions behind and if this iPad JUST got upgraded to the lastest iOS 12 from a much, MUCH older version of iOS ( the 2013 iPad Air 1 started out on iOS 7 ), then there maybe nothing more to be done.

You are extremely late in upgrading/updating this iPad model.

Those files maybe irretrievable now and gone forever.

Sorry.