You may find that while opening a pdf file, the file name is different from the one you save as, this article will help.
When you find an involved pdf file, please open the file in Foxit PDF Editor, go to File>Properties>Description, check if the Title name is same with the one you get.
And go to File>Properties>Initial View, you can find the option "Display Document Title" has been checked, it will set the title name as default name while opening.
Generally the title name is added while creating the pdf file. For example, if your pdf file is created from Microsoft Office Word , specially Word 2019, the original word files have the title name, title name will be set as the default file name while converting.
Now there are two workarounds for you to fix the issue.
1. While creating the pdf file from Microsoft Office Word
Before convert to pdf file, you can open the word file in Microsoft Office Word, go to File > Info > Properties /Advanced Properties and delete the title that is there. It then defaults to the file name, not the title name in the pdf files.
If you do not want to change the properties of the word file, while creating pdf file from Office Word by Foxit PDF add-in, you can click Advanced, and uncheck " Preserve Document Properties", then the created pdf file will not contains document properties including file title.
2. But for the created pdf file
You can open it in Foxit PDF Editor, go to File>Properties, uncheck 'Display Document Title', save the change, and reopen your pdf file.
Also, you can batch change the option for the existing pdf files by File>Action Wizard.