Properly Copy Associated PDFs Between .bib Files

[rioxddd]

Describe the changes you have made here: This PR is intended to resubmit the proposed changes to the correct repository, as the previous PR submission koppor#700 was mistakenly placed elsewhere.
Will try to update the code as you guided.
Link the issue that will be closed: (koppor#520)

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
• Change in CHANGELOG.md described in a way that is understandable for the average user (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.