PDF stands for Portable Document Format. Files that contain the .pdf file extension are older Adobe Acrobat files. The newer .pdf extension has replaced the older .pdf version.
The .pdf_ file extension used to be used when creating PDF documents on older Apple computers. PDF files allow documents to be transmitted over the Internet and viewed across multiple platforms, regardless of the application that the original document was created in.
The newer PDF documents that are created with recent versions of the software and with the versions created for Windows computers are saved with the .pdf file suffix rather than the .pdf_ file extension.
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If you have additional information about the PDF_ file format or software that uses files with the PDF_ suffix, please do get in touch - we would love hearing from you.