How to Download a Document as Base64 Using the BoldSign API

You can retrieve a document in Base64 format by adding the x-response-format header with the value base64 to your request. When enabled, the API returns the document as a Base64‑encoded string along with its content type and file name.

Example cURL Request (Base64 Response)

curl -X GET 'https://api.boldsign.com/v1/document/download?documentId=YOUR_DOCUMENT_ID' \
  -H 'accept: application/json' \
  -H 'X-API-KEY: {your-api-key}' \
  -H 'x-response-format: base64'

Header Parameter

x-response-formatstring Set this to base64 to receive the document content as a Base64‑encoded string.

The response also includes a fileName field, which reflects the generated file name based on the uploaded document. By default, this includes the document title and its status (for example: Contract_Completed.pdf).

For more details, refer to: https://support.boldsign.com/kb/article/18128/customize-signed-document-naming

Base64 Response Format

200 – Success

{
  "fileName": "Contract_Completed.pdf",
  "contentType": "application/pdf",
  "file": "JVBERi0xLjcNCiVi...base64encodedstring...ZCVFDQo="
}

Response Fields:

fileName – The name of the downloaded file
contentType – The MIME type of the document (e.g., application/pdf)
file – The Base64‑encoded content of the file

Other Endpoints Supporting Base64 Format

The x-response-format: base64 header can also be used with the following endpoints:

DownloadAuditTrail
DownloadAttachment
Template Download endpoints

All of these endpoints follow the same Base64 retrieval pattern.