Yes, it is possible to link cloud documents of Leads with Opportunity and Account  on Lead conversion. Follow the steps below:

  • From Setup, search for flows in the Quick Find Box, and click on New Flow.
  • Select Record-triggered Flow and click on Create
  • Select Lead as the object.
  • Trigger the flow when a record is updated.
  • Set entry conditions to: All conditions are Met, IsConverted = TRUE.
  • For "When to Run the Flow for Updated Records", choose "Every time a record is updated and meets the condition requirements."
  • Optimize the Flow for Actions and Related Records.
  • Click on Done.
  • From Elements, drag the Get Records element to the canvas.
  • It will open a New Popup. 
  • For Object, select DragDropToCloud__cloud_Documents__c object.
  • For Filter choose None-Get All Records
  • For How many records to store select All Records
  • For How to Store Record Data select Automatically store all fields
  • For the label, give a name of your choice.
  • API Name field will be populated automatically.
  • From Elements, drag the Decision element to the canvas.
  • It will open a New Popup. 
  • For the label, give a name of your choice.
  • API Name field will be populated automatically.
  • For outcome enter Label and  API Name field will be populated automatically.
  • For Condition Requirement to Execute outcome select All Conditions are Met(AND)
  • Check if the Get All CD records (the Get Records element from the previous step) is not null.
  • For When to execute outcome select If the condition requirements are met
  • Click on Done
  • From Elements, drag the Update Records element to the canvas.
  • For the label give a name as per your choice
  • API Name field will be populated automatically.
  • How to Find Records to Update and Set their Values Choose - Update records related to the lead record that triggered the flow
  • Select Related Record - Record.Cloud_Documents__r
  • Set Filter Conditions - None Update all related Records.
  • Set Field Values for the Cloud Documents Records - Set below 2 fields:

                      1. Account__c = Record.Converted Account ID

                      2. Opportunity__c = Record.Converted Opportunity ID

  • Click on Done.
  • Connect elements of flow as shown in the below image.
  • Save and Activate the Flow.


PS: View Drag, Drop & Upload Files to Amazon S3, a Salesforce AppExchange app for moving files from Salesforce to Amazon S3: https://appexchange.salesforce.com/appxListingDetail?listingId=a0N3000000B5ZPqEAN