AmazonS3 with the Standard storage classis the best solution:

Encryption: SSE-S3 ensures server-side encryption of the data, meeting compliance requirements.

Immediate access: The Standard storage class provides low-latency and high-throughput access to data.

Multi-location storage: Cross-Region replication ensures data is stored in multiple AWS Regions for redundancy.

Why Other Options Are Not Ideal:

Option B:

Amazon EFS is more costly and suited for file systems rather than object storage.Not cost-effective.

Option C:

Amazon EBS is block storage and not optimized for object storage like PDFs. Backup schedules do not ensure immediate availability.Not suitable.

Option D:

S3 Glacier Flexible Retrieval is designed for archival, not immediate access.Does not meet access requirements.

AWS References:

Amazon S3 Standard Storage:AWS Documentation - S3 Storage Classes

Amazon S3 Cross-Region Replication:AWS Documentation - Cross-Region Replication

AWS Encryption Options:AWS Documentation - S3 Encryption