You need to spend time with those libraries to know more about its features. Here some Open source libraries
Open Source PDF Libraries in C#[
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Regarding your question ... Basically you need to use Header. Report Document has Header & Footer which is basically for repeated contents like Logo, Title, Page Number, Date-Time, etc., So you have to add your text in Header section. I'm sure most libraries have this option because Header & Footer are important parts of Reports.
I found below articles
Generating Dynamic PDF Documents using the Open Source Scryber Library[
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PDF File Writer C# Class Library (Version 1.28.0)[
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Search Codeproject & Web for more articles.