First of all, download and open Delphi OOo, an excellent toolbox to use Delphi with LibreOffice. You can find Delphi OOo here.

Then, use this piece of code to open a template, search and replace text and generate a PDF file:

...

fileProp3, fileProp4, oReplace, myTemplate: Variant;

myFile3, myFile4: String;

...

myFile3:= convertToURL('D:\template.ott');

fileProp3:= CreateProperties(['AsTemplate', true]);

myTemplate := StarDesktop.LoadComponentFromURL(myFile3, '_blank', 0, fileProp3);

oReplace := myTemplate.createReplaceDescriptor;

oReplace.setSearchString('field1');

oReplace.setReplaceString('New content!');

myTemplate.ReplaceAll(oReplace);

fileProp4:= CreateProperties(['FilterName', 'writer_pdf_Export']);

myFile4:= convertToURL('C:\new_document.pdf');

myTemplate.storeToURL(myfile4, fileProp4);

Delphi 7,  LibreOffice 3.4.2, Windows XP SP 3.