Você precisa contar seus colchetes - as duas consultas que estão sendo unidas devem estar em um colchete - então seu alias e SELECT * FROM funcionarão.
SELECT *
FROM ( (SELECT DISTINCT (doc_hdr.doc_hdr_id),
doc_hdr.initr_prncpl_id,
doc_hdr.doc_hdr_stat_cd,
doc_hdr.crte_dt,
doc_hdr.ttl,
doc_hdr.app_doc_stat,
doc1.doc_typ_nm,
doc1.lbl,
doc1.doc_hdlr_url,
doc1.actv_ind
FROM krew_doc_typ_t doc1, krew_doc_hdr_t doc_hdr
WHERE doc_hdr.initr_prncpl_id IN ('10000000001')
AND (doc1.doc_typ_nm = 'PO'
OR doc1.doc_typ_nm = 'POA'
OR doc1.doc_typ_nm = 'POC'
OR doc1.doc_typ_nm = 'POPH'
OR doc1.doc_typ_nm = 'PORH'
OR doc1.doc_typ_nm = 'POR'
OR doc1.doc_typ_nm = 'PORT'
OR doc1.doc_typ_nm = 'POSP'
OR doc1.doc_typ_nm = 'POV')
AND doc_hdr.doc_hdr_stat_cd != 'I'
AND doc_hdr.doc_typ_id = doc1.doc_typ_id)
UNION
SELECT DISTINCT doc_hdr.doc_hdr_id,
doc_hdr.initr_prncpl_id,
doc_hdr.doc_hdr_stat_cd,
doc_hdr.crte_dt,
doc_hdr.ttl,
doc_hdr.app_doc_stat,
doc1.doc_typ_nm,
doc1.lbl,
doc1.doc_hdlr_url,
doc1.actv_ind
FROM krew_doc_typ_t doc1, krew_doc_hdr_t doc_hdr
WHERE doc1.doc_typ_nm = 'PO'
AND CAST (doc_hdr.doc_hdr_id AS VARCHAR (14)) =
ANY (SELECT ppt.fdoc_nbr
FROM pur_po_t ppt, pl_purchase_log_po_t ppl
WHERE ppt.ap_pur_doc_lnk_id =
ANY (SELECT prt.ap_pur_doc_lnk_id
FROM pur_reqs_t prt,
krew_doc_hdr_t doc_hdr
WHERE CAST (
doc_hdr.doc_hdr_id AS VARCHAR (14)
) = prt.fdoc_nbr
AND doc_hdr.initr_prncpl_id IN
('10000000001'))))
final_search
ORDER BY final_search.doc_hdr_id DESC;
