diff --git a/app.py b/app.py
index ab66e30..ab86a9d 100644
--- a/app.py
+++ b/app.py
@@ -16,12 +16,19 @@ from functools import wraps
 import secrets
 import base64
 import hashlib
+from flask_session import Session
 
 app = Flask(__name__)
 app.config['UPLOAD_FOLDER'] = tempfile.mkdtemp()
 app.config['SECRET_KEY'] = os.getenv('SECRET_KEY', 'secret')
 app.config['MAX_CONTENT_LENGTH'] = 1000 * 1024 * 1024 * 1024  # 1000TB
 
+# Configure server-side sessions to avoid cookie size limits
+app.config['SESSION_TYPE'] = 'filesystem'
+app.config['SESSION_FILE_DIR'] = '/tmp/flask_sessions'
+app.config['SESSION_PERMANENT'] = False
+Session(app)
+
 # Load environment variables
 load_dotenv()
 
diff --git a/requirements.txt b/requirements.txt
index 897ba73..3e77fe9 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -1,4 +1,5 @@
 Flask==3.0.0
+Flask-Session==0.6.0
 PyMuPDF==1.23.7
 Werkzeug==3.0.1
 waitress==3.0.0
diff --git a/ubuntu-install.sh b/ubuntu-install.sh
index 87e8bb8..7611dc6 100644
--- a/ubuntu-install.sh
+++ b/ubuntu-install.sh
@@ -328,6 +328,11 @@ sudo chown $USER:$USER /var/log/pdf-sort-merge
 log_info "Creating application data directories..."
 mkdir -p results static templates
 
+# Create Flask session directory for server-side sessions
+log_info "Creating Flask session directory..."
+sudo mkdir -p /tmp/flask_sessions
+sudo chown $USER:$USER /tmp/flask_sessions
+
 # Set up firewall (optional)
 log_info "Configuring firewall..."
 sudo ufw allow 80/tcp || log_warning "Failed to configure firewall for port 80 - you may need to do this manually"