$path = 'E:/Zl/资料-1/资料库/3005  奥迪A6L新型C6原厂资料';
function get_filetree($path){
    $tree = array();
    foreach(glob($path."/{*.pdf,*.doc,*.ppt}", GLOB_BRACE) as $single){
        if(is_dir($single)){
            $tree = array_merge($tree,get_filetree($single));
        }
        else{
            $tree[] = $single;
        }
    }
    return $tree;
}
print_r(get_filetree($path));

function get_filetree($path){
  $tree = array();
  foreach(glob($path."/*.{pdf,doc,ppt}", GLOB_BRACE) as $single) {
    if(is_file($single)) $tree[] = $single;
  }
  foreach(glob($path."/*", GLOB_ONLYDIR ) as $single) {
    $tree = array_merge($tree,get_filetree($single));
  }
  return $tree;
}

------解决方案--------------------

你#6的写法也是可以的，但用错了目标
if(glob($path."*/*.{pdf,doc,ppt}", GLOB_BRACE)){
应为
if(glob($single."{pdf,doc,ppt}", GLOB_BRACE)){
目的是判断 $single 的后缀是否符合要求

我#4的代码：
 //取得所有符合条件的文件名
 foreach(glob($path."/*.{pdf,doc,ppt}", GLOB_BRACE) as $single) {
   if(is_file($single)) $tree[] = $single;
 }
 //遍历所有子目录
 foreach(glob($path."/*", GLOB_ONLYDIR ) as $single) {
   $tree = array_merge($tree,get_filetree($single));
 }