Use the GetDocuments request to retrieve all Documents in a Polarion project.
PolarionClient client = PolarionClient.createClient(serverUri, username, password);
URI queryUri = new GetServiceProviderCatalog(client).call()
.getServiceProvider(projectId)
.getQueryCapability(Polarion.TYPE_DOCUMENT)
.getQueryBase();
List<Document> documents = new GetDocuments(client, queryUri).call();
To retrieve Work Items contained in a Document, use GetDocumentWorkItems:
List<WorkItem> workItems = new GetDocumentWorkItems(client, documentUri).call();