win7 ran livekd,but reported "cannot verify the digital signature"

Question

I have downloaded the ntoskrnl.pdb，and set the path "_NT_SYMBOL_PATH".
When I typed the "livekd -k "xx\yy\windbg(x64)\kd.exe" -vsym"
it reported that "Windows无法验证此文件的数字签名。某软件或硬件最近有所更改，可能安装了签名错误或损毁的文件，或者安装的文件可能是来路不明的恶意软件。"
It meant that "Windows cannot verify the digital signature of this file. A piece of software or hardware has recently changed, there may be a incorrectly signed or corrupted file installed, or the installed file may be malicious software of unknown origin."