We don't talk here about paste options, but of the fact, that a big number is represented in a scientific notation, and there it seem nothing to do about it in Excel actually. Please see the OP, but also the images I posted above.
Images are not helpful and only lead to misinterpretations of what is actually happening during data processing. We need your file, or a sample file, to show you what kind of data you really have and what happens.
Unfortunately, explanations are lengthy and the technical details cannot be explained in 2 sentences.
Maybe a short video helps:

In order for you to understand this, you need to know that numbers are only processed up to 15 digits in Excel and almost all programs in the world. Scientific notation for numbers with more digits is also common.
The key point, and I know this is confusing, is that you can only represent larger numbers as text in Excel. And all other programs on this planet.
And unfortunately, the cell format is also involved here, which leads to even more confusion.
For us as humans, a number is a number because we see it on screen. But in Excel, a number on screen, can mean in fact that you have text OR a number in a cell.
And now you also need to know that changing the cell format does NOT affect the cell content.
A text remains as text, regardless of the cell format.
A number remains as number, regardless of the cell format.
Even if both look like a number to you.
Alright, are you still with me? If so upload you file here:
Microsoft Answers Community Public Request - Dropbox
Or upload it on any file hoster of your choice and post the download link here.
Then I'll look at your file and show you how it works.
Andreas.