Federal Statutes
The federal statutes are published in a number of places--the most typical citation you would need to find will be from the U.S. Code, which is the formal codification of the federal statutes currently in-force. Occasionally, you may also be asked to find copies of slip laws (Public Laws) or session laws (U.S. Statutes at Large), which are statutes at an earlier stage in the publication process (i.e. really recent laws before they've been incorporated into the existing text of the U.S. Code).
Historic Federal Statutes
In some instances, you may need an older copy of a federal statute to substantiate what the law was at some point in the past. As noted above, the govinfo.gov coverage is better for relatively recent materials so to find older versions you will need to look elsewhere.
State Statutes
Pursuant to The Bluebook's Rule 12.5(b) Internet and online sources:
So in many instances, you should be able to cite to an html version rather than requiring a pdf or scan from the print.
To help find the relevant sources, typically it is a good idea to start by locating a Research Guide to the law of that state simply by doing an online search for "[State] legal research guide." Often the best guides will be run by the law librarians of the state's flagship law school.
Some states may have authenticated pdf copies of state codes available through the state legislature's website. You may be able to find these resources by first identifying the name of a state's codified statute set by checking T1 of the Bluebook, then doing an internet search for that statute set.
Historic State Statutes
There are several useful resources for finding pdfs of historic state statutes, though the coverage is highly variable both by jurisdiction and by included date ranges.
Case Law
If you are looking for an authenticated pdf of a court opinion, it is usually best to start your search in Westlaw as they often will include a link to the original pdf near the top of the case. Using that pdf should be one of the most efficient ways of obtaining the case.
For federal cases, West Publishing publishes the Supreme Court Reporter (SCOTUS), the Federal Reporter (U.S. Courts of Appeals), the Federal Supplement (federal District Courts), the Federal Appendix (unreported federal cases).
West Publishing's Regional Reporter series includes most of the preferred reporters for State supreme court decisions.
Westlaw should make the original pdf images available for all cases published in any of these reporters.
For United States Supreme Court decisions, you will need to use the preferred reporter--the United States Reports. Fortunately this title is available through HeinOnline's collections and can be accessed at the link below.
Historic Case Law
In rare instances where you need to find historic case law or copies of older state reporters, there are several resources available, though the coverage varies by jurisdiction. If you are unable to locate something using these resources, please contact a librarian to explore additional options.
Recent Dockets and Court Filings
Typically if you are looking to find dockets or court filings such as briefs or motions from relatively recent litigation, you will be looking at PACER ("Public Access to Court Electronic Records") or Bloomberg Law for federal filings or state court websites for state court filings. It's worth noting too that sometimes Westlaw and Lexis will have copies of some of the filings for a case available in the "Filings" tab (Westlaw) or the Shepard's Report (Lexis) for a given case, so it is often worth checking those resources first.
As with most things, finding federal court filings is much easier than finding state court filings. When it comes to finding state court filings, note that different states, and sometimes different courts within the states, all have different requirements for electronic filing, so there can be a lot of variability.
Historic Dockets and Court Filings
If you are looking for older court filings from cases that predated electronic filing requirements, there are a number of places to turn. As usual, these work better for federal cases and dockets. For state materials, you may need to contact the Clerk of Courts for the deciding court to figure out what their retention schedule is and if the records have been preserved or moved to an archival storage space.
Federal Regulations
There are several different government websites that provide access to authenticated pdfs of both the Code of Federal Regulations (CFR) and the Federal Register (FR). One of the best is govinfo.gov, mentioned above as a source for pdfs of the official U.S. Code and statutes. The CFR collects all the federal regulations currently in-force and organizes them topically into separate titles. The Federal Register is the daily publication of the federal Executive branch and includes proposed and final regulations, notices, guidance documents, and executive orders, in addition to other items.
Historic Federal Regulations
If you need to find an older copy of the Code of Federal Regulations as evidence, perhaps, of what the regulations were at a specific point in the past, there are several resources to consult.
Federal Legislative History
For Federal legislative history documents (bills, committee reports, committee hearings, etc.), there are a number of great resources for finding official, authenticated pdf versions--though it is important to pay attention to scope and the specific kinds of documents included.
State Legislative History
State legislative history documents can be extremely difficult to find, especially because some states do not even generate or preserve legislative history. It is always best to start by identifying a state-specific legislative history legal research guide by doing an online search. The best resources are likely to come from the state's flagship law school or perhaps a state government agency like a legislative reference bureau or state supreme court library. This will help you figure out where certain items might be available.
With more recent legislation, state legislature websites can be a valuable resource for material. Often bills, committee reports, or recordings of hearings may be linked out from the legislature's website.