Pdf string detection with yara rule not working

Question

Hi everyone I have a pdf file word1.pdf which include hello_1234 and some other text. I want to detect all pdf file having string hello-1234. But the yara rule doesn't detect pdf with hello wird but it works for txt file.

My yara rule is

Rule example1{
Strings:
    $Str1 = {25 50 44 46} // for pdf 
detection 
    $str2 = "hello-1234" // string in pdf 
    //$str2 = /hello-\d{4}/ // regex for string detection 

Condition:
    $str1 and $str2

}

Cmd command 

Yara64 filename.yara C:/directory of pdf files.

Output 
Dose not gives any output