Word does a lot of extra formatting that doesn't necessarily move cleanly into HTML.  Therefore, I usually paste text only and then re-do the formatting once I move it into the HTML.  However, once or twice I have done this when I really wanted it to match Word:

1. In the Word Document, select File > Save As.
2. For the type, select "Web Page, Filtered".
3. Save the file somewhere easy to locate.
4. Now find the file you saved and open it in a text editor (I like Notepad++, but Windows comes with Notepad if you don't want to download something extra).
5. Copy everything.
6. Paste into the HTML tab on the Custom HTML element in LFForms.
7. Move to the Visual tab and click into the content box.  It should take a second or two and refresh the preview of the field.  It's actually removing some of the tags that were there from Word which would be for a full page (head and body tags for example).
8. Once you've confirmed the preview loaded, you can make any tweaks you need and/or click Done.