Lost format after pasting

Hi all

I have two files: let's say File1 and File2.File1 has one column: datas are in a text format.I copy this column. I want to paste it in my second file.File2 is closed.I open it and i paste my column.
Here's my problem. If i let my first file opened, there's no problem(!!): my column is pasted as i want. My datas keep their original format.
But, if i close my first file and after, i open File2 and i paste my column, then my datas lose their original format.My column takes a numeric format.I can see it because my datas are on the left in the column.And i can chek it by a sum: if i do 400+200, for example, the result is 600. In the first case(text format), the result is 0.I don't understand why.If someone can explain me this situation, i'd be happy.I repeat: my second file is closed. I open it after my copy in the first file.

Thanks in advance for your help.