Best implementation approach to download large numbers of files in given time limit? [closed]

I need some guidance points to decide best approaches to implement solution on bellow given use case:

We have an Angular UI application which has download PDF files functionality. This UI app sends a download REST request to Java Spring Boot middleware app which calls third party/downstream application to get PDF files.

The number of files that are sent from UI in the request (list of pdfIds) to download varies based on the user we have logged-in and can be ranged form 1 to 1000 or more. Current implementation is that if we click on download button then all the pdfIds belongs to that user are sent for download and based on number of files it takes number of seconds to download from downstream systems. For example, if we have 10 files, it completes zip download of 10 files in 1 sec, if we have 200 files it takes 20 secs likewise.

The issue is that we have a mandatory request timeout bound of 10 secs, if request is taking more than 10 secs, it will throw request timeout in UI. So, I need the best approach to resolve this without modifying timeout bound. Solution can be front-end (Angular) or back-end (Java) oriented.

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