var rtf = File.ReadAllText(fileName);
rtf = this.RenameStyleDifferentInOnlySpaces(rtf);
var document = provider.Import(rtf);

...

private string RenameStyleDifferentInOnlySpaces(string rtf)
{
    HashSet<string> styles = new HashSet<string>();

    string pattern = @"{\\(?:\*\\c)?s([0-9]+)[^}]*\n?[^}]*\\[^' ]* ?'?([^;]*)";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string styleName = match.Groups[2].Value.Replace(" ", string.Empty);
        if (styles.Contains(styleName))
        {
            styleName = this.ReplaceOldStyleName(ref rtf, styles, match).Replace(" ", string.Empty);
        }

        styles.Add(styleName);
    }

    return rtf;
}

private string ReplaceOldStyleName(ref string rtf, HashSet<string> styles, Match match)
{
    string oldStyleName = match.Groups[2].Value;
    StringBuilder styleNameBuilder = new StringBuilder(oldStyleName + "0");
    while (styles.Contains(styleNameBuilder.ToString().Replace(" ", string.Empty)))
    {
        styleNameBuilder.Append("0");
    }

    string oldMatch = match.Groups[0].Value;
    string newMatch = oldMatch.Replace(oldStyleName, styleNameBuilder.ToString());
    rtf = rtf.Replace(oldMatch, newMatch);
    return styleNameBuilder.ToString();
}

RtfFormatProvider provider = new RtfFormatProvider();
var rtf = File.ReadAllText(ofd.FileName);
rtf = this.ReplaceSelfBasedOnStyle(rtf);
this.flowDocument = provider.Import(rtf);

...

private string ReplaceSelfBasedOnStyle(string rtf)
{
    string pattern = @"{[\n]*\\s[0-9]+[^;]* \\\w* (Normal);}";

    var matches = Regex.Matches(rtf, pattern);
    foreach (Match match in matches)
    {
        string oldValue = match.Value;
        string newValue = oldValue.Replace(@"\sbasedon0 ", string.Empty);

        rtf = rtf.Replace(oldValue, newValue);
    }

    return rtf;
}

private void CleanParagraphsBeforeTablesOnNewPage()
{
    List<Paragraph> paragraphs = this.flowDocument.EnumerateChildrenOfType<Paragraph>().ToList();
    foreach (var paragraph in paragraphs)
    {
        BlockContainerBase parent = (BlockContainerBase)paragraph.Parent;
        int paragraphIndex = parent.Blocks.IndexOf(paragraph);
        int blocksCount = parent.Blocks.Count;

        bool isAfterPageBreak = paragraphIndex > 0 && this.PreviousBlockEndsWithPageBreak(parent.Blocks[paragraphIndex - 1]);

        int nextIndex = paragraphIndex + 1;
        bool nextBlockIsTable = nextIndex < blocksCount && parent.Blocks[nextIndex] is Table;

        if (isAfterPageBreak && nextBlockIsTable)
        {
            parent.Blocks.Remove(paragraph);
        }
    }
}

private bool PreviousBlockEndsWithPageBreak(BlockBase blockBase)
{
    bool isLastInlinePageBreak = false;
    bool isParagraph = blockBase is Paragraph;
    if (isParagraph)
    {
        Paragraph paragraph = (Paragraph)blockBase;
        InlineBase lastInline = paragraph.Inlines.Last();

        bool isBreak = lastInline is Break;
        if (isBreak)
        {
            isLastInlinePageBreak = ((Break)lastInline).BreakType == BreakType.PageBreak;
        }
    }

    return isLastInlinePageBreak;
}