There may be a problem with the downloader code, causing the PDF file to be corrupted

There may be a problem with the downloader code, causing the PDF file to become corrupted
The PDF file in the server is healthy: Verified by direct FTP download.
Some of the code for the downloader is as follows:

$file _size = filesize ($filedir);
Header ("content-type:application/octet-stream;charset=iso-8859-1");
Header ("Cache-control:private");
Header ("Accept-ranges:bytes");
Header ("Accept-length:". $this->fjsize);
Header ("content-disposition:attachment; Filename= ". $this->delfilename);
$fp = fopen ($filedir, "R");
$buffer _size = 1024;
$cur _pos = 0;
while (!feof ($fp) && $filedir-$cur _pos > $buffer _size)
{
$buffer = Fread ($fp, $buffer _size);
Echo $buffer;
$cur _pos + = $buffer _size;
}

if ($file _size > 0)
$buffer = Fread ($fp, $file _size-$cur _pos);
Else
$buffer = Fread ($fp, $this->fjsize);
Echo $buffer;
Fclose ($FP);
Flush ();
------Solution--------------------
$fp = fopen ($filedir, "R b"); Binary mode open, although Linux is not required, but still recommended to do so
Text Open file, 0x1a is considered the end of the file

while (!feof ($fp) && $filedir-$cur _pos > $buffer _size)
Writing
while (!feof ($FP))
Feof ($FP) is the end of the file.
Again $filedir-$cur _pos > $buffer _size no need
Double judgments can lead to errors, such as $filedir not $buffer _size integer times

In addition, you cannot export anything other than the contents of the file, including the BOM header

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