Then, that is, with certainty, the issue.
The version of Pages on that aging iPad is too old to open Pages documents that have been edited on a more recent version of Pages, either on a more recent iOS device with a more recent version of the Pages app or on a Mac with a newer version of the Pages app.
Older versions of any Apple Pages app cannot open an edited Pages app edited with a newer version of the Pages ( iWork ) apps.
Sorry, but there is no more to be done.
Any iOS device that is stuck on iOS12.5.5, is too old, now, and is unsupported by Apple and third party app support is very quickly dwindling away, in 2022.
These iOS 12 devices are, now, almost 8 and 9-year old devices, in 2022.
Sorry.
It is mid 2022, now.
If you need/want an iPad with an up to date iPadOS with access to current, updated apps, then you really need to seriously consider a purchase of a new iPad model OR a much, MUCH “newer” and less expensive, refurbished or used iPad models, from better known, trusted sources, like other Apple product/device retailers/resellers, some locally or from online, on the Web/Internet, capable of running the latest iPadOS versions.
There are PLENTY of newer, older iPad models that can run the latest, current versions of iPadOS.
If you need OR ONLY able to purchase older, less expensive iPad models, look at older IPads from 2015 and later.
All of these older model iPads below CAN, currently, run the latest releases of iPadOS 15 and the up and coming iPadOS 16, releasing sometime in the Fall of 2022.
2015-2017, 12.9 inch screen iPad Pro models. ( 1st gen Pencil support )
2017, 10.5 inch screen iPad Pro models ( 1st gen Pencil support )
2017, 9.7 inch screen iPad 5th gen models ( NO Pencil support )
2018, 9.7 inch screen iPad 6th gen models ( 1st gen Pencil support )
2019, 10.2 inch screen iPad 7th gen models ( 1st gen Pencil support )
2020, 10.2 inch screen iPad 8th gen models. ( 1st gen Pencil support )
2018, 11 inch or 12.9 inch screen iPad Pro models ( 2nd gen Pencil support )
2019, 10.5 inch screen iPad Air 3rd gen models ( 1st gen Pencil support )
2019, 7.9 inch screen iPad Mini 5th gen models ( 1st gen Pencil support )
Get the largest internal data storage model you can afford as NO Apple iOS device has ANY expandable storage.
I am sorry that you had to learn all of this, but this does not change any of the facts regarding these older iOS devices.
Sorry & Best of Luck to You!