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Documents on electronic media need a unique file name to be identified. Usually this consists of the actual name and an extension (.doc for Word documents, .jpg for images, etc.) The file name becomes unique, however, only in association with the URL directory-path leading to the file. In image SEO, the file name is an important ranking criterion for Google’s image search.
An example of a path might look like this:
Depending on the operating system, file names can consist of different character sets (usually ASCII or Unicode) and be based on different nomenclatures. Letters that cannot be used (such as “ö,” “ü” or “&”), must be replaced by circumlocutions (z. B. "oe").
The maximum length of a filename typically includes 256 characters.
Filename in SEO
As the Internet is largely builds on Unix computers, the following rules for file names have become prevalent and they can be understood by Googlebot:
- Any special characters are displayed URL encoded
- Instead of spaces dashes ( "-") are inserted
- These characters are reserved and therefore not used: # $% & '() * +, /:; =? @ 
- Case sensitivity is avoided, everything is lowercase.
To avoid common errors during file retrieval, special characters should be avoided when saving files. Likewise, it is not advisable to use the tilde in the file name.
Significance for search engine optimization
The file name plays an important role especially for image optimization. Using keywords in file names is a must, so that the image will be listed well in the Google image search under the applicable keywords. Thus, webmasters or SEOs should consider how to name the image files before integrating them into a CMS.
If your domain gets transferred or relaunched, the file names or file paths must be adjusted to allow the retrieval of images on the website without any broken links resulting in the corresponding images not getting displayed. And just as is the case for websites that are unreachable, the rankings for the respective images can crash because of the wrong file paths.
If you want to rank well with a PDF, you should include applicable keywords and optimize anchor texts when creating the file name.