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Detecting if a PDF has been edited can be done through several methods:

1. Checking Metadata
- File Properties: Open the PDF and go to File > Properties. Under the Description tab, you can see the creation and modification dates. If the modification date is later than the creation date, the file has likely been edited.
- Advanced Metadata: Some PDF editors, like Adobe Acrobat, allow you to view more detailed metadata, which can include information about the software used for editing and the times of edits.

2. Using Digital Signatures
- Digital Signatures: If a PDF is digitally signed, any changes made after the signature was applied will invalidate the signature. This can be checked in the Signatures panel of Adobe Acrobat.

3. Comparing Versions
- Comparison Tools: Tools like Adobe Acrobat Pro DC and Wondershare PDFelement offer a comparison feature. You can compare the edited PDF with the original version to highlight any changes.

4. Visual Inspection
- Manual Comparison: If you have a copy of the original PDF, you can manually compare it with the edited version to spot any differences. This can be time-consuming but effective for minor changes.

5. Using Specialized Software
- Forensic Tools: There are specialized forensic tools designed to detect alterations in PDF files. These tools can analyze the file structure and metadata to identify any inconsistencies.

Considerations
- Limitations: Metadata can be altered, and not all changes are easily detectable. For critical documents, using digital signatures and keeping original copies are the best practices.