Merge PDFs and add a password in C# (.NET)

What you need

• An API key. Create one in the app under API keys and export it as FORMFEED_API_KEY. A test key (ff_test_…) is right for trying things out. See the quick start.
• C# (.NET). You need the .NET 8 SDK; the program uses System.Net.Http.Json and System.Text.Json, which ship with it.
• The sources. sources.json next to the program: a JSON array of what goes into the document, in order. A render id (rnd_…) or the name of a PDF in the workspace library, two to fifty of them.
• A password. In FORMFEED_PDF_PASSWORD. Generate one per document and send it through a different channel than the file.
• A plan with PDF tools. The tools are part of Starter and above.

dotnet new console -o render && cd render   # then replace Program.cs

The program

The program reads the sources, merges them into one PDF, encrypts that result with the password and saves the finished file.

using System.Net.Http.Headers;
using System.Net.Http.Json;
using System.Text.Json;

using var http = new HttpClient { BaseAddress = new Uri("https://api-eu.formfeed.dev/v1/") };

// one call to the API: the key goes on each request, never on the client, so the download does not carry it
async Task<JsonElement> Api(string path, object body)
{
    using var request = new HttpRequestMessage(HttpMethod.Post, path) { Content = JsonContent.Create(body) };
    request.Headers.Authorization = new AuthenticationHeaderValue("Bearer", Environment.GetEnvironmentVariable("FORMFEED_API_KEY"));
    var response = await http.SendAsync(request);
    var answer = await response.Content.ReadFromJsonAsync<JsonElement>();
    if (!response.IsSuccessStatusCode)
    {
        Console.Error.WriteLine($"{answer.GetProperty("title")}: {(answer.TryGetProperty("detail", out var d) ? d : "")}");
        Environment.Exit(1);
    }
    return answer;
}

var sources = JsonSerializer.Deserialize<string[]>(await File.ReadAllTextAsync("sources.json"));

// 1. join the documents, in the order they are given
var merged = await Api("pdf/merge", new { sources, filename = "bundle.pdf" });

// 2. encrypt the result; printing stays allowed, copying and editing do not
var protectedPdf = await Api("pdf/protect", new
{
    source = merged.GetProperty("id").GetString(),
    user_password = Environment.GetEnvironmentVariable("FORMFEED_PDF_PASSWORD"),
    permissions = new[] { "print" },
});

await File.WriteAllBytesAsync("bundle.pdf", await http.GetByteArrayAsync(protectedPdf.GetProperty("download_url").GetString()));
Console.WriteLine($"{protectedPdf.GetProperty("id")} {protectedPdf.GetProperty("page_count")} page(s), {protectedPdf.GetProperty("units")} unit(s)");

Run it

It prints the render’s id, the page count and the units, and bundle.pdf lies next to the program. Opening it asks for the password; printing works, copying text does not.

export FORMFEED_API_KEY=ff_test_...
export FORMFEED_PDF_PASSWORD=$(openssl rand -base64 18)
dotnet run

What is particular about C# (.NET)

• Anonymous objects. The API’s field names are snake_case and are written as they are (webhook_url = …), so no naming policy has to be configured.
• Data without classes. JsonDocument.Parse(…).RootElement embeds the sample data untouched; in your application this is your own record or DTO.
• Key on the request, not the client. The Authorization header is set on each request to the API, never on the HttpClient: the download goes to a signed CDN URL and must not carry your API key.

Why two calls

• Order matters. An encrypted PDF cannot be changed any more, so the password comes last. Merging an already protected document fails.
• Every step is a render. Merge and protect each produce a render of their own with an id, a download URL and a line in the render log; the second takes the first’s id as its source.
• Half a unit each. A tool costs 0.5 units on a live key, whatever the page count. The renders that produced the sources were counted when they ran.
• Or in one request. A render can carry the same steps as post and do everything in one call. Use the tools when the documents already exist, and post when you are rendering anyway. See PDF tools.

What the permissions do

Everything not listed in permissions is forbidden (leave the list out and all four of print, copy, modify and annotate are allowed): with ["print"] a reader may print but not copy text, edit or annotate. Without a user_password anyone can open the document and the permissions still apply, which is the right setting for a document that is public but should not be edited. An owner_password lifts the restrictions; a random one is used when you send none. PDF viewers honour the permissions, but they are no protection against someone who can open the file: confidentiality comes from the user_password.

When the API says no

• One shape for every error. Errors are problem documents (RFC 9457) with type, title, status, detail, a stable code and the request_id to quote to support; the type URL leads to a page about that error. The list is in the error reference.
• The usual ones. 401 unauthorized for a missing or revoked key, 404 template_not_found for a wrong template name, 422 template_runtime_error when the template fails on the data, 429 rate_limited above the plan’s requests per second, 402 quota_exceeded when the Free plan’s units are used up (a paid plan bills extra units up to its hard cap).
• Retry safely. Send an Idempotency-Key header with the POST, and repeat the request with the same key when you do not know whether the first attempt arrived: for 24 hours the key returns the first answer instead of rendering and billing again. The answer is stored once the first attempt has been answered, so do not retry while it may still run; and problems below 500 are stored too, so a corrected request needs a new key.

The job around this program

This guide is one program. What the job needs around it, from the template to the limits and the price, is on the page Contract and agreement PDFs.

More C# (.NET) guides

• Generate a PDF invoice from JSON in C# (.NET)
• Create a ZUGFeRD / Factur-X e-invoice in C# (.NET)
• Convert HTML to PDF in C# (.NET)
• Generate Open Graph images in C# (.NET)
• Generate PDFs from a CSV in C# (.NET)
• Async PDF rendering with a verified webhook in C# (.NET)

FAQ

Where do the render ids come from?

From the renders you made earlier: the id of every answer. They must belong to the same workspace and their output must still exist. Files you uploaded to the library are named instead.

Which encryption is used?

AES-256, done by qpdf. The password never appears in the render log, and it is not stored: lose it and the document cannot be opened.

Can I watermark as well?

Yes, /pdf/watermark is the third tool and takes a text or an image. Chain it before the password, like the merge. See PDF tools.

Merge and protect in other languages

• Node.js (TypeScript)
• Python
• PHP
• Go
• Java
• Ruby
• curl