Have you been asked to provide a particular document not only in PDF format, but specifically in PDF/A? You are in luck as you can easily create PDF/A with LibreOffice.
As you probably know, PDF is a standard for encoding documents that is portable between systems. Then, if you are wondering how PDF/A differs from “conventional” PDF, according to Wikipedia:

PDF/A is an ISO-standardized version of the Portable Document Format (PDF) specialized for use in the archiving and long-term preservation of electronic documents. PDF/A differs from PDF by prohibiting features ill-suited to long-term archiving, such as font linking (as opposed to font embedding) and encryption. The ISO requirements for PDF/A file viewers include color management guidelines, support for embedded fonts, and a user interface for reading embedded annotations.

For such reasons PDF/A is frequently required by corporations and governments whom often have to preserve files for a long time.

How to create PDF/A with LibreOffice

As the title imply this guide works with LibreOffice, but it should work also with OpenOffice. In case you are using a different office suite, since both suites are free you can download them and use alongside it.
The procedure to create PDF/A with LibreOffice is quite simple. With your document open, go to “Files” and select “Export as PDF …”.

In the window that pop up select “Archive PDF/A-1a (ISO 19005-1)” and then “Export”.

In the next windows select where to save the file and its name and you are done. The generated PDF conform to the PDF/A standard.
As a bonus information, if you have to convert to PDF/A an existing PDF file the procedure is the same; you just have to open the existing PDF file with LibreOffice by going to “File” -> “Open…” and follow the above procedure.