If I copy text from an outside source, as in a .pdf, pretty much any outside source, and paste it into writer, it changes the page format, in particular the margin settings get set to 1.25" all around. If I copy from another .odt file and paste, it does not create the problem. This was not an issue in the 3.4.5 and earlier releases.
Not only does pasting from a pdf mess with the margins, if you try to paste from a pdf in between existing text, it will paste it at the end of the file instead. Also some pdf text pasted becomes stubbornly formatted, making clear format useless as it turns it into large bold characters. Basically whatever new code was introduced for pasting in the releases since the grammar checker was introduced is buggy.
I had the same problem with version 220.127.116.11 (x86) on x64 Windows 7.
I have managed to solve it by disabling the experimental features (Tools/Options/LibreOffice/General/Enable experimental (unstable) features). The problem seems to appear when using Paste Special/Formatted text [RTF], no matter the source (mine was a CSV opened in Calc, not even another software).
*** This bug has been marked as a duplicate of bug 46669 ***