diff --git a/papers/commands/__init__.py b/papers/commands/__init__.py
index 0076b49..bde2380 100644
--- a/papers/commands/__init__.py
+++ b/papers/commands/__init__.py
@@ -9,3 +9,4 @@ import edit_cmd
 import remove_cmd
 import websearch_cmd
 import tags_cmd
+import attach_cmd
diff --git a/papers/commands/attach_cmd.py b/papers/commands/attach_cmd.py
new file mode 100644
index 0000000..b0d8eb3
--- /dev/null
+++ b/papers/commands/attach_cmd.py
@@ -0,0 +1,34 @@
+from .. import repo
+from .. import configs
+from .helpers import (add_references_argument, parse_reference,
+                      add_docfile_to_paper)
+
+
+def parser(subparsers, config):
+    parser = subparsers.add_parser('attach',
+            help='attach a document to an existing paper')
+    parser.add_argument('-c', '--copy', action='store_true', default=None,
+            help="copy document files into library directory (default)")
+    parser.add_argument('-C', '--nocopy', action='store_false', dest='copy',
+            help="don't copy document files (opposite of -c)")
+    add_references_argument(parser, single=True)
+    parser.add_argument('document', help='pdf or ps file')
+    return parser
+
+
+def command(config, ui, copy, reference, document):
+    """
+    :param bibfile: bibtex file (in .bib, .bibml or .yaml format.
+    :param docfile: path (no url yet) to a pdf or ps file
+    """
+    if copy is None:
+        copy = config.get(configs.MAIN_SECTION, 'import-copy')
+    rp = repo.Repository.from_directory(config)
+    key = parse_reference(ui, rp, reference)
+    paper = rp.get_paper(key)
+    try:
+        add_docfile_to_paper(rp, paper, docfile=document, copy=copy)
+    except ValueError, v:
+        ui.error(v.message)
+        ui.exit(1)
+# TODO handle case where citekey exists
diff --git a/papers/commands/helpers.py b/papers/commands/helpers.py
index 586f64d..c857fd3 100644
--- a/papers/commands/helpers.py
+++ b/papers/commands/helpers.py
@@ -1,6 +1,7 @@
 from .. import files
 from .. import color
 from ..repo import InvalidReference
+from ..paper import NoDocumentFile
 
 
 def add_references_argument(parser, single=False):
@@ -12,14 +13,18 @@ def add_references_argument(parser, single=False):
                 help="one or several reference to export (citekeysor numbers)")
 
 
+def add_docfile_to_paper(repo, paper, docfile, copy=False):
+    if copy:
+        repo.import_document(paper.citekey, docfile)
+    else:
+        paper.set_external_document(docfile)
+        repo.add_or_update(paper)
+
+
 def add_paper_with_docfile(repo, paper, docfile=None, copy=False):
     repo.add_paper(paper)
     if docfile is not None:
-        if copy:
-            repo.import_document(paper.citekey, docfile)
-        else:
-            paper.set_external_document(docfile)
-            repo.add_or_update(paper)
+        add_docfile_to_paper(repo, paper, docfile, copy=copy)
 
 
 def extract_doc_path_from_bibdata(paper, ui):
diff --git a/papers/commands/list_cmd.py b/papers/commands/list_cmd.py
index 03d2953..02cd9a5 100644
--- a/papers/commands/list_cmd.py
+++ b/papers/commands/list_cmd.py
@@ -6,7 +6,7 @@ from .. import color
 def parser(subparsers, config):
     parser = subparsers.add_parser('list', help="list papers")
     parser.add_argument('-k', '--citekeys-only', action='store_true',
-            default=False,
+            default=False, dest='citekeys',
             help='Only returns citekeys of matching papers.')
     parser.add_argument('query', nargs='*',
             help='Paper query (e.g. "year: 2000" or "labels: math")')
diff --git a/papers/papers b/papers/papers
index 1851170..55281b8 100755
--- a/papers/papers
+++ b/papers/papers
@@ -21,6 +21,7 @@ cmds = collections.OrderedDict([
         ('open', commands.open_cmd),
         ('websearch', commands.websearch_cmd),
         ('tags', commands.tags_cmd),
+        ('attach', commands.attach_cmd),
         ])
 
 config = configs.read_config()