PDF - Remove White Margins

问题

I would like to know a way to remove white margins from a PDF file. Just like Adobe Acrobat X Pro does. I understand it will not work with every PDF file.

I would guess that the way to do it, is by getting the text margins, then cropping out of that margins.

PyPdf is preferred.

iText finds text margins based on this code:

public void addMarginRectangle(String src, String dest)
    throws IOException, DocumentException {
    PdfReader reader = new PdfReader(src);
    PdfReaderContentParser parser = new PdfReaderContentParser(reader);
    PdfStamper stamper = new PdfStamper(reader, new FileOutputStream(RESULT));
    TextMarginFinder finder;
    for (int i = 1; i <= reader.getNumberOfPages(); i++) {
        finder = parser.processContent(i, new TextMarginFinder());
        PdfContentByte cb = stamper.getOverContent(i);
        cb.rectangle(finder.getLlx(), finder.getLly(),
            finder.getWidth(), finder.getHeight());
        cb.stroke();
    }
    stamper.close();
}

回答1:

I'm not too familiar with PyPDF, but I know Ghostscript will be able to do this for you. Here are links to some other answers on similar questions:

Convert PDF 2 sides per page to 1 side per page (SuperUser.com)
Freeware to split a pdf's pages down the middle? (SuperUser.com)
Cropping a PDF using Ghostscript 9.01 (StackOverflow.com)

The third answer is probably what made you say 'I understand it will not work with every PDF file'. It uses the pdfmark command to try and set the /CropBox into the PDF page objects.

The method of the first two answers will most likely succeed where the third one fails. This method uses a PostScript command snippet of <</PageOffset [NNN MMM]>> setpagedevice to shift and place the PDF pages on a (smaller) media size defined by the -gNNNNxMMMM parameter (which defines device width and height in pixels).

If you understand the concept behind the first two answers, you'll easily be able to adapt the method used there to crop margins on all 4 edges of a PDF page:

An example command to crop a letter sized PDF (8.5x11in == 612x792pt) by half an inch (==36pt) on each of the 4 edges (command is for Windows):

gswin32c.exe ^
    -o cropped.pdf ^
    -sDEVICE=pdfwrite ^
    -g5400x7200 ^
    -c "<</PageOffset [-36 -36]>> setpagedevice" ^
    -f input.pdf

The resulting page size will be 7.5x10in (== 540x720pt). To do the same on Linux or Mac, use:

gs \
    -o cropped.pdf \
    -sDEVICE=pdfwrite \
    -g5400x7200 \
    -c "<</PageOffset [-36 -36]>> setpagedevice" \
    -f input.pdf

Update: How to determine 'margins' with Ghostscript

A comment asked for 'automatic' determination of the white margins. You can use Ghostscript's too for this. Its bbox device can determine the area covered by the (virtual) ink on each page (and hence, indirectly the whitespace for each edge of the canvas).

Here is the command:

gs \
  -q -dBATCH -dNOPAUSE \
  -sDEVICE=bbox \
   input.pdf

Output (example):

 %%BoundingBox: 57 29 562 764
 %%HiResBoundingBox: 57.265030 29.347046 560.245045 763.649977
 %%BoundingBox: 57 28 562 667
 %%HiResBoundingBox: 57.265030 28.347046 560.245045 666.295011

The bbox device renders each PDF page in memory (without writing any output to disk) and then prints the BoundingBox and HiResBoundingBox info to stderr. You may modify this command like that to make the results more easy to parse:

gs \
    -q -dBATCH -dNOPAUSE \
    -sDEVICE=bbox \
     input.pdf \
     2>&1 \  
  | grep -v HiResBoundingBox

Output (example):

 %%BoundingBox: 57 29 562 764
 %%BoundingBox: 57 28 561 667

This would tell you...

...that the lower left corner of the content rectangle of Page 1 is at coordinates [57 29] with the upper right corner is at [562 741]
...that the lower left corner of the content rectangle of Page 2 is at coordinates [57 28] with the upper right corner is at [561 667]

This means:

Page 1 uses a whitespace of 57pt on the left edge (72pt == 1in == 25,4mm).
Page 1 uses a whitespace of 29pt on the bottom edge.
Page 2 uses a whitespace of 57pt on the left edge.
Page 2 uses a whitespace of 28pt on the bottom edge.

As you can see from this simple example already, the whitespace is not exactly the same for each page. Depending on your needs (you likely want the same size for each page of a multi-page PDF, no?), you have to work out what are the minimum margins for each edge across all pages of the document.

Now what about the right and top edge whitespace? To calculate that, you need to know the original page size for each page. The most simple way to determine this: the pdfinfo utility. Example command for a 5 page PDF:

pdfinfo \
  -f 1 \
  -l 5 \
   input.pdf \
| grep "Page "

Output (example):

Page    1 size: 612 x 792 pts (letter)
Page    2 size: 612 x 792 pts (letter)
Page    3 size: 595 x 842 pts (A4)
Page    4 size: 842 x 1191 pts (A3)
Page    5 size: 612 x 792 pts (letter)

This will help you determine the required canvas size and the required (maximum) white margins of the top and right edges of each of your new PDF pages.

These calculations can all be scripted too, of course.

But if your PDFs are all of a uniq page size, or if they are 1-page documents, it all is much easier to get done...