diff --git a/.travis.yml b/.travis.yml
index 98f81ad..690e557 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -4,13 +4,14 @@ os:
 language: python
 python:
     - 3.7
+    - 3.8
 
 install:
     - python setup.py install
 
 script:
     - echo 268642 > /tmp/test.txt
-    - nhentai --cookie "_ga=GA1.2.2000087053.1558179358; __cfduid=d8930f7b43d04e1b2117719e28386b2e31593148489; csrftoken=3914GQGSmmqQyfQTBswNgfXuhFiefu8sAgOnsfZWiiqS4PJpKivuTp34p2USV6xu; sessionid=be0w2lwlprlmld3ahg9i592ipsuaw840"
+    - nhentai --cookie "_ga=GA1.2.1651446371.1545407218; __cfduid=d0ed34dfb81167d2a51a1d6392c1768a81601380350; csrftoken=KRN0GR1ft86m3HTefpQA99pp6R1Bo7hUs5QxNGOAIuwB5g4EcJj04fwMB8QKgLaB; sessionid=7hzoowox78c90wi5ud5ibphm4axcck7c"
     - nhentai --search umaru
     - nhentai --id=152503,146134 -t 10 --output=/tmp/ --cbz
     - nhentai -F
diff --git a/nhentai/__init__.py b/nhentai/__init__.py
index e622c47..5561dc3 100644
--- a/nhentai/__init__.py
+++ b/nhentai/__init__.py
@@ -1,3 +1,3 @@
-__version__ = '0.4.0'
+__version__ = '0.4.2'
 __author__ = 'RicterZ'
 __email__ = 'ricterzheng@gmail.com'
diff --git a/nhentai/downloader.py b/nhentai/downloader.py
index 2d9a0f4..f11bb60 100644
--- a/nhentai/downloader.py
+++ b/nhentai/downloader.py
@@ -5,11 +5,10 @@ import multiprocessing
 import signal
 
 from future.builtins import str as text
+import sys
 import os
 import requests
-import threadpool
 import time
-import multiprocessing as mp
 
 try:
     from urllib.parse import urlparse
@@ -18,10 +17,10 @@ except ImportError:
 
 from nhentai.logger import logger
 from nhentai.parser import request
-from nhentai.utils import Singleton, signal_handler
+from nhentai.utils import Singleton
 
 requests.packages.urllib3.disable_warnings()
-semaphore = mp.Semaphore()
+semaphore = multiprocessing.Semaphore(1)
 
 
 class NHentaiImageNotExistException(Exception):
@@ -133,16 +132,14 @@ class Downloader(Singleton):
         queue = [(self, url, folder) for url in queue]
 
         pool = multiprocessing.Pool(self.size, init_worker)
-
-        for item in queue:
-            pool.apply_async(download_wrapper, args=item, callback=self._download_callback)
+        [pool.apply_async(download_wrapper, args=item) for item in queue]
 
         pool.close()
         pool.join()
 
 
 def download_wrapper(obj, url, folder=''):
-    if semaphore.get_value():
+    if sys.platform == 'darwin' or semaphore.get_value():
         return Downloader.download_(obj, url=url, folder=folder)
     else:
         return -3, None
diff --git a/nhentai/parser.py b/nhentai/parser.py
index 97c1b15..93dd109 100644
--- a/nhentai/parser.py
+++ b/nhentai/parser.py
@@ -178,7 +178,6 @@ def doujinshi_parser(id_):
     doujinshi['img_id'] = img_id.group(1)
     doujinshi['ext'] = ext
 
-    pages = 0
     for _ in doujinshi_info.find_all('div', class_='tag-container field-name'):
         if re.search('Pages:', _.text):
             pages = _.find('span', class_='name').string
diff --git a/nhentai/utils.py b/nhentai/utils.py
index f9ae776..e8626b9 100644
--- a/nhentai/utils.py
+++ b/nhentai/utils.py
@@ -9,7 +9,6 @@ import zipfile
 import shutil
 import requests
 import sqlite3
-import img2pdf
 
 from nhentai import constant
 from nhentai.logger import logger
@@ -195,6 +194,11 @@ def generate_cbz(output_dir='.', doujinshi_obj=None, rm_origin_dir=False, write_
 
 
 def generate_pdf(output_dir='.', doujinshi_obj=None, rm_origin_dir=False):
+    try:
+        import img2pdf
+    except ImportError:
+        logger.error("Please install img2pdf package by using pip.")
+
     """Write images to a PDF file using img2pdf."""
     if doujinshi_obj is not None:
         doujinshi_dir = os.path.join(output_dir, doujinshi_obj.filename)
@@ -233,12 +237,6 @@ and append a file extension like '.txt', so I avoid the potential of using
 an invalid filename.
 
 """
-    if sys.platform.startswith('win32'):
-        invalid_chars = '\/:*?"<>|.'
-        for char in invalid_chars:
-            s = s.replace(char, '_')
-    return s
-
     # maybe you can use `--format` to select a suitable filename
     valid_chars = "-_.()[] %s%s" % (string.ascii_letters, string.digits)
     filename = ''.join(c for c in s if c in valid_chars)
diff --git a/requirements.txt b/requirements.txt
index e34829d..3fbad9d 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -4,5 +4,4 @@ BeautifulSoup4>=4.0.0
 threadpool>=1.2.7
 tabulate>=0.7.5
 future>=0.15.2
-img2pdf>=0.3.6
 iso8601 >= 0.1
diff --git a/setup.py b/setup.py
index 7b87d5c..8218604 100644
--- a/setup.py
+++ b/setup.py
@@ -23,7 +23,7 @@ setup(
 
     author=__author__,
     author_email=__email__,
-    keywords='nhentai, doujinshi',
+    keywords=['nhentai', 'doujinshi', 'downloader'],
     description='nhentai.net doujinshis downloader',
     long_description=long_description(),
     url='https://github.com/RicterZ/nhentai',