"Unreadable content" warning in Word document when exporting from PDF

I am using the API to generate a .docx file from a PDF.

When I use this online converter tool, everything works perfectly: https://www.adobe.com/acrobat/online/pdf-to-word.html

However when I use the API with the same PDF, my exported .docx file is corrupted. When I go to open the file in Word, I get the following message:

"Word found unreadable content in "Untitled Document.docx". Do you want to recover the contents of this document? If you trust the source of this document, click Yes."

If I click "Yes", the file opens just fine. But I can't figure out what could be going wrong for Word to be giving me this message.

Here is my code for sending the request:

headers = {
 'Authorization': "Bearer #{access_token}",
 'Accept': 'application/json, text/plain, */*',
 'x-api-key': client_id,
 'Prefer': "respond-async,wait=0",
 'Content-Type': "multipart/form-data"
}

j = {
 "cpf:engine": {
  "repo:assetId": "urn:aaid:cpf:Service-26c7fda2890b44ad9a82714682e35888"
 },
 "cpf:inputs": {
  "params": {
   "cpf:inline": {
    "targetFormat": "docx"
   }
  },
  "documentIn": {
   "dc:format": "application/pdf",
   "cpf:location": "InputFile0"
  }
 },
 "cpf:outputs": {
  "documentOut": {
   "dc:format": "application/vnd.openxmlformats-officedocument.wordprocessingml.document",
   "cpf:location": "multipartLabelOut"
  }
 }
}

body = {
 "contentAnalyzerRequests": j.to_json,
 "InputFile0": file.open
}

resp = post(url, body, headers)