Merge the documents returned from a set of specified data loaders. 
from langchain_community.document_loaders import WebBaseLoader

loader_web = WebBaseLoader(
    "https://github.com/basecamp/handbook/blob/master/37signals-is-you.md"
)

from langchain_community.document_loaders import PyPDFLoader

loader_pdf = PyPDFLoader("../MachineLearning-Lecture01.pdf")

from langchain_community.document_loaders.merge import MergedDataLoader

loader_all = MergedDataLoader(loaders=[loader_web, loader_pdf])

docs_all = loader_all.load()

len(docs_all)

23