Are all PDF files compressed?

问题:

So there are some threads here on PDF compression saying that there is some, but not a lot of, gain in compressing PDFs as PDFs are already compressed.

My question is: Is this true for all PDFs including older version of the format?

Also I'm sure its possible for someone (an idiot maybe) to place bitmaps into the PDF rather than JPEG etc. Our company has a lot of PDFs in its DBs (some older formats maybe). We are considering using gzip to compress during transmission but don't know if its worth the hassle

回答1:

PDFs in general use internal compression for the objects they contain. But this compression is by no means compulsory according to the file format specifications. All (or some) objects may appear completely uncompressed, and they would still make a valid PDF.

There are commandline tools out there which are able to decompress most (if not all) of the internal object streams (even of the most modern versions of PDFs) -- and the new, uncompressed version of the file will render exactly the same on screen or on paper (if printed).

So to answer your question: No, you cannot assume that a gzip compression is adding only hassle and no benefit. You have to test it with a representative sample set of your files. Just gzip them and take note of the time used and of the space saved.

It also depends on the type of PDF producing software which was used...

回答2:

Instead of applying gzip compression, you would get much better gain by using PDF utilities to apply compression to the contents within the format as well as remove things like unneeded embedded fonts. Such utilities can downsample images and apply the proper image compression, which would be far more effective than gzip. JBIG2 can be applied to bilevel images and is remarkably effective, and JPEG can be applied to natural images with the quality level selected to suit your needs. In Acrobat Pro, you can use Advanced -> PDF Optimizer to see where space is used and selectively attack those consumers. There is also a generic Document -> Reduce File Size to automatically apply these reductions.

Update:

Ika's answer has a link to a PDF optimization utility that can be used from Java. You can look at their sample Java code there. That code lists exactly the things I mentioned:

• Remove duplicated fonts, images, ICC profiles, and any other data stream.
• Optionally convert high-quality or print-ready PDF files to small, efficient and web-ready PDF.
• Optionally down-sample large images to a given resolution.
• Optionally compress or recompress PDF images using JBIG2 and JPEG2000 compression formats.
• Compress uncompressed streams and remove unused PDF objects.