Is there any way to pre-populate documents in file upload field?

Yes, it is possible.
When you are uploading a document for the first time in a file upload field, update a rule input say ri!doc with the document id of the uploaded file. You can do that in the saveInto parameter. Similarly, you will have rule inputs for all other document upload fields say ri!doc1, ri!doc2,etc.
Now, for all other file upload fields, in the value parameter, check if rule input corresponding to that file upload field is null say isnull(ri!doc1), isnull(ri!doc2), etc.. If it is null, make the value as ri!doc else keep it as ri!doc1....

Thanks for the solution.
I tried this way. But this brings a new issue: "Once I upload a document for the first time, it gets auto populated to all other rows. But when I try to update any row with a new document, only the updated document gets saved." I found this error is due to the same document ID existence for all the fields being auto populated.

But what if the first file (ri!doc) is just uploaded to the form which is not yet submitted. which means ri!doc will carry a temporary document ID. In such case, when I delete any of the auto-populated rows, the temporary document id pointing to the document gets deleted. Hence the first document is not getting saved whenever it is updated in any other rows.

But what if the first file (ri!doc) is just uploaded to the form which is not yet submitted. which means ri!doc will carry a temporary document ID. In such case, when I delete any of the auto-populated rows, the temporary document id pointing to the document gets deleted. Hence the first document is not getting saved whenever it is updated in any other rows.