Detect if PDF file is correct (header PDF) [closed]

问题

I have a windows .NET application that manages many PDF Files. Some of the files are corrupt.

2 issues: I'll try to explain in my imperfect English...sorry

1.)

How can I detect if any pdf file is correct ?

I want to read header of PDF and detect if it is correct.

var okPDF = PDFCorrect(@"C:\temp\pdfile1.pdf");

2.)

How to know if byte[] (bytearray) of file is PDF file or not.

For example, for ZIP files, you could examine the first four bytes and see if they match the local header signature, i.e. in hex

50 4b 03 04

if (buffer[0] == 0x50 && buffer[1] == 0x4b && buffer[2] == 0x03 && buffer[3] == 0x04)

If you are loading it into a long, this is (0x04034b50). by David Pierson

I want the same for PDF files.

byte[] dataPDF = ...

var okPDF = PDFCorrect(dataPDF);

Any sample source code in .NET?

回答1:

1) Unfortunately there is no easy way to determine is pdf file corrupt. Usually the problem files have a correct header so the real reasons of corruption are different. PDF file is effectively a dump of PDF objects. The file contains a reference table giving the exact byte offset locations of each object from the start of the file. So most probably corrupted files have a broken offsets or may be some object is missed.

The best way to determine that the file is corrupted is to use specialized PDF libraries. There are lots of both free and commercial of such libraries for .NET. You may simply try to load PDF file with one of such libraries. iTextSharp will be a good choice.

2) In accordance with PDF reference the header of PDF file has usually form %PDF−1.X (where X is a number, for the present from 0 to 7). And 99% of PDF files have such header. But also there are some other kinds of headers which Acrobat Viewer accepts and even absence of header isn't real problem for PDF viewers. So you shouldn't treat file as corrupted if it hasn't a header. E.g. the header may be appeared somewhere within the first 1024 bytes of the file or be in the form %!PS−Adobe−N.n PDF−M.m

Just for your information I am a developer of the Docotic PDF library.

回答2:

I check Header PDF like this:

 public bool IsPDFHeader(string fileName)
    {
        byte[] buffer = null;
        FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
        BinaryReader br = new BinaryReader(fs);
        long numBytes = new FileInfo(fileName).Length;
        //buffer = br.ReadBytes((int)numBytes);
        buffer = br.ReadBytes(5);

        var enc = new ASCIIEncoding();
        var header = enc.GetString(buffer);

        //%PDF−1.0
        // If you are loading it into a long, this is (0x04034b50).
        if (buffer[0] == 0x25 && buffer[1] == 0x50
            && buffer[2] == 0x44 && buffer[3] == 0x46)
        {
            return header.StartsWith("%PDF-");
        }
        return false;

    }

回答3:

The first line of a PDF file is a header identifying the version of the PDF specification to which the file conforms %PDF-1.0, %PDF-1.1, %PDF-1.2, %PDF-1.3, %PDF-1.4 etc.

You could check this by reading some bytes from the start of the file and see if you have the header at the beginning for a match as PDF file. See the PDF reference from Adobe for more details.

Don't have a .NET example for you (haven't touched the thing in some years now) but even if I had, I'm not sure you can check for a complete valid content of the file. The header might be OK but the rest of the file might be messed up (as you said yourself, some files are corrupt).

回答4:

Well-behaving PDFs start with the first 9 Bytes as %PDF-1.x plus a newline (where x in 0..8). 1.x is supposed to give you the version of the PDF file format. The 2nd line are some binary bytes in order to help applications (editors) to identify the PDF as a non-ASCIItext file type.

However, you cannot trust this tag at all. There are lots of applications out there which use features from PDF-1.7 but claim to be PDF-1.4 and are thusly misleading some viewers into spitting out invalid error messages. (Most likey these PDFs are a result of a mis-managed conversion of the file from a higher to a lower PDF version.)

There is no such section as a "header" in PDF (maybe the initial 9 Bytes of %PDF-1.x are what you meant with "header"?). There may be embedded a structure for holding metadata inside the PDF, giving you info about Author, CreationDate, ModDate, Title and some other stuff.

My way to reliably check for PDF corruption

There is no other way to check for validity and un-corrupted-ness of a PDF than to render it.

A "cheap" and rather reliable way to check for such validity for me personally is to use Ghostscript.

However: you want this to happen fast and automatically. And you want to use the method programatically or via a scripted approach to check many PDFs.

Here is the trick:

Don't let Ghostscript render the file to a display or to a real (image) file.
Use Ghostscript's nullpage device instead.

Here's an example commandline:

gswin32c.exe ^
    -o nul ^
    -sDEVICE=nullpage ^
    -r36x36 ^
    "c:/path to /input.pdf"

This example is for Windows; on Unix use gs instead of gswin32c.exe and -o /dev/null.

Using -o nul -sDEVICE=nullpage will not output any rendering result. But all the stderr and stdout output of Ghostscript's processing the input.pdf will still appear in your console. -r36x36 sets resolution to 36 dpi to speed up the check.

%errorlevel% (or $? on Linux) will be 0 for an uncorrupted file. It will be non-0 for corrupted files. And any warning or error messages appearing on stdout may help you to identify problems with the input.pdf.

There is no other way to check for a PDF file's corruption than to somehow render it...

Update: Meanwhile not only %PDF-1.0, %PDF-1.1, %PDF-1.2, %PDF-1.3, %PDF-1.4, %PDF-1.5, %PDF-1.6, %PDF-1.7 and %PDF-1.8 are valid version indicators, but also %PDF-2.0.

回答5:

You could use iTextSharp to open and attempt to parse the file (e.g. try and extract text from it) but that's probably overkill. You should also be aware that it's GNU Affero GPL unless you purchase a commercial licence.

回答6:

Checking the header is tricky. Some of the code above simply won't work since not all PDF's start with %PDF. Some pdf's that open correctly in a viewer start with a BOM marker, others start like this

------------e56a47d13b73819f84d36ee6a94183 Content-Disposition: form-data; name="par" ...etc

So checking for "%PDF" will not work.

回答7:

What I do is:

1.Validate extension

2.Open PDF file, read the header (first line) and check if it contains this string: "%PDF-"

3.Check if the file contains a string that specifies the number of pages by searching for multiple "/Page" (PDF file should always have at least 1 page)

As suggested earlier you can also use a library to read the file: Reading PDF File Using iTextSharp

来源：https://stackoverflow.com/questions/3108201/detect-if-pdf-file-is-correct-header-pdf

标签

file

pdf

header

corrupt