When I transfer the text of the Word file to Libre Office, a gray painted column remains between some words. How to get rid of it?
No, there is no gray space between words as you can see in the print preview. A grey portion of text is some kind of text field with a special meaning. While in edit view, text fields are displayed with a grey background in order to make them distinguishable from regular text content, otherwise you could not understand how the document works.
There is no grey shade when you view it in print preview, when you turn off edit mode (Ctrl+Shift+M) or when you export the document as pdf.
This gray-background space is not an ordinary U+0020 SPACE character but a U+00A0 NO-BREAK SPACE which was created by pressing some modifier key simultaneously with the space bar. As you didn’t mention your OS, I can’t be more specific.
Such a non-breaking space doesn’t behave like an ordinary space and seriously impacts line break positions and justification.
To get rid of them, copy one in Edit
>Find & Replace
dialog with an ordinary space in the replacement box. Press Replace All.
In the future, avoid pressing any other key when you hit the space bar.