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Some sites are known as "clean" and are set to open all PDF files, although most sites are set to open PDF files with filtering of the text and of the pictures.
PDF filtering works by processing the contents of the file and extracting the images from it, replacing them with a blurred image and checking their content if the image is new or has not yet been approved.
If it is a previously checked image that has been disapproved, the image is replaced with the blocked symbol. An image that has not been checked yet, is sent to be checked (assuming that the "embedded images for testing" setting is enabled by default),if the image is approved it will appear after the test by re-downloading the file .
As a result of image filtering, sometimes the file gets corrupted. Because of that every time you open a pdf file you receive an alert that the file may be corrupted. This behavior can be overridden by checking the "Do not warn of file tampering" option in the "Personal Filter Settings".