Are pdf files compressed?

[yoda313]

I've been wondering about something for awhile now. At work I use pdf files all the time for emailing paper copies. I've noticed that quite often a file will be reported as larger than it really is when it hits the printer queue. Is this because pdf files are compressed? I mean a 1mb pdf file will sometimes read as 2mb or larger as it is printing. Even small ones like a .5mb will read as much larger than the stated file size on the pc. So are they compressed?

[rallynavvie]

PDF files can be compressed when they're created using any number of compression methods depending on the content. For instance pictures can be compressed within a PDF using something like JPEG compression. Text data can be converted to line art so that the font doesn't need to be "packaged" with the PDF for viewing on a PC that may not have that particular font.

However I believe what you're seeing is the file being rasterized before printing. You PDF files may have been created directly from native files on a PC, for instance Word, and contain text data and/or line art in addition to any photos that may be included. Printers don't draw in lines, they draw with tiny little dots, so they can't always interpret the line data from fonts or line art on its own. Rasterizing turns the line art and text into all those tiny little dots for the printer to understand. Raster images generally occupy more space than vector data (line art). Some printers have their rasterizing process done on a controller on the printer itself, though most rely on the PC to do this for them when the document is printed.

You'll probably see the same thing with printing from something like Word or Publisher. The document size may only be 500kb but when printing it balloons out to 2MB.

[yoda313]

Ok. I think I follow that. Thanks rallynavvie.