A listener named Hernán wrote that he was writing about Word documents and PDF documents or files. He said, “I am struggling to know the proper way to address the name of these items; is it ‘Word document’ or ‘word document’? Is it ‘.pdf’ document with a period, or just ‘pdf,’ or all capital ‘PDF’?” He also asked how to talk about multiple PDF files. In other words, how to make “PDF” plural.
These are great questions. I’ve had to look up the answers myself in the past.
According to the Chicago Manual of Style, the names of computer programs, operating systems, and so on are capitalized and written without quotations. For example, you’d write that you created a Word document (with “Word” capitalized and “document” lowercase).
Chicago also says that you write file formats in all caps, so you would then write that you converted your Word document to a PDF or a PDF file.
The F in “PDF” doesn’t stand for “file.” It stands for “format” in “portable document format” so “PDF file” isn’t redundant.
If you have two, they are PDFs (with “PDF” in all caps and then a lowercase S at the end).
The AP Stylebook is less clear about these kinds of abbreviations, but it does say to use all caps for “JPEG,” “GIF,” “PDF,” and “MP3.” On the other hand, it recommends lowercase for “zip files” and lowercase if you have to quote someone talking about a “.exe” file, for example.
Thanks for the question, Hernán. I hope that helped!
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Mignon Fogarty is Grammar Girl and the founder of Quick and Dirty Tips. Check out her New York Times bestseller, “Grammar Girl’s Quick and Dirty Tips for Better Writing.”