Design and Architecture

1. Lazy load the sub tabs. However, some tabs cannot be loaded unless data from other tabs is there. Other tabs are stand-alone.
2. Everything is synchronous. Make back end calls async.
3. If a tab's data is changed, for the most part the other tabs don't know about it. A few raise a changed event, but for the most part, the Project needs to be reloaded. Changes from other users are not reflected at all until the Project is reloaded. So I could keey the local data synchronized, but for data from other users, provide change notification somehow.

//Main Form:
private async void toolTrim_Click(object sender, EventArgs e)
{
    var form = new TrimOptions();
    DialogResult result = form.ShowDialog();
    if (result == DialogResult.OK)
    {
        await Task.Run(() => filesList.TrimPages(form.trimFirst, form.trimLast, form.ifBlank));
    }
    form.Dispose();
}

//filesList Class:
public void TrimPages(bool trimFirst, bool trimLast, bool ifBlank)
{
    for (int entry = 0; entry < TotalPDFs; entry++)
    {
        FileEntry thisFile = list[entry];
        thisFile.Trim(trimFirst, trimLast, ifBlank);
    }
}

//thisFile Class:
public void Trim(bool front, bool back, bool ifBlank)
{
    // code
}

//Main Form:
private async void toolTrim_Click(object sender, EventArgs e)
{
    await Task.Run(() => filesList.TrimPages());
}

//filesList Class:
public void TrimPages(bool trimFirst, bool trimLast, bool ifBlank)
{
    var form = new TrimOptions();
    DialogResult result = form.ShowDialog();
    if (result == DialogResult.OK)
	{
		for (int entry = 0; entry < TotalPDFs; entry++)
		{
			PDFEntry thisFile = list[entry];
		}
	}
}

//thisFile Class:
public void Trim(bool front, bool back, bool ifBlank)
{
    // code
}