It doesn't clear the metadata of your documents for you
Numerous files format store hidden data or metadata inside of the files. Text processors or PDF files could store the name of the author, the date and time of creation of the file, and sometimes even parts of the editing history of the file… Those hidden data depend on the file format and the software used.
Images file formats, like TIFF of JPEG, probably take the prize in this field. Those files, created by digital cameras or mobile phones, contain a metadata format called EXIF which can include the date, time and sometimes the GPS coordinates of the picture, the brand and serial number of the device which took it as well as a thumbnail of the original image. Image processing software tend to keep those data intact. Internet is full of cropped or blurred images for which the EXIF thumbnail still contains the full original picture.
Tails doesn't clear the metadata of your files for you. Yet. Still it's in Tails' design goal to help you do that. For example, Tails already comes with Exiv2, an image metadata manipulation tool.
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http://tails.boum.org/doc/warning/index.en.html
