想请教各位大神，我想使用copy循环下载网上的文件
local p 19991
while `p'>19989 {
copy "https://www.nber.org/papers/w`p'.pdf" w`p'.pdf, replace
local p=`p'-1
}

运行总是报错
sun.security.validator.ValidatorException: PKIX path building failed: sun.security.provider.certpath.SunCertPathBuilde
> rException: unable to find valid certification path to requested target
r(5100);


第一次运行好像还没问题，现在就总是报错了。这该怎么解决？