You can do that using a query like this. Please note that you must change the /site/doc_lib_name search text to match your doc library FileDirRef
spjs.ac.textField({
"applyTo": "your_field_name",
"helpText": "Start typing",
"loadText": "Searching...",
"listGuid": "your_doc_lib_name_or_guid",
"listBaseUrl": _spPageContextInfo.webServerRelativeUrl,
"showField": "FileLeafRef",
"searchFields": ["FileLeafRef"],
"filterCAML": "",
"useREST": true,
"preloadData": true,
"filterREST": "FSObjType eq 1 and FileDirRef eq '/site/doc_lib_name'",
"optionDetailFields": [],
"optionDetailPrefix": [],
"enforceUniqueValues": false,
"rowLimit": 15,
"listOptionsOnFocus": true,
"minLengthBeforeSearch": 1,
"reValidateOnLoad": true,
"allowAddNew": true,
"isLookupInSelf": false,
"addNewAdditionalFields": [],
"multiselect": false,
"multiselectSeparator": "; ",
"orderBy": [],
"clearSetFieldsOnInvalidSelection": true,
"setFields": [],
"debug": false
});
Alexander