Online-convert.com supports the following text converter: DOCX to TXT, DOC to TXT, ODT to TXT, PDF to TXT, SXW to TXT, WPD to TXT, RTF to TXT and HTML to TXT (Experimental).

OCR function, that is to say recognization of text in an image, isn't available.

Test converting pdf-files to txt-files: no OCR-function in difference to Free File Converter
I converted 2 pdf-documents to doc-documents:
(A) pdf with markable and searchable text, and (B) a scanned pdf-file with text not markable or searchable.
pdf-document (A) has been converted to a normal doc-document with text with very good quality, pdf-document (B) has been converted to a doc-document with an page-filling image for each page, in the same way as Free File Converter (1) did it with the 2 examined documents. Then I exported a pdf-file with good readable text to a png-file and to a gif-file with good readable text.
The converting of these 2 grafic-files to a txt-file resulted in the error:
"There has been an error converting your file. Please take a look at our FAQ."
Free File Converter can also OCR-convert png-, pdf- and gif-files to txt-files:
Then I converted these 2 grafical-files to a txt-file using Free File Converter (1). The result was a OCR-converted text-file in all 2 cases. However, if the font is small, the OCR-recognition of Free File Converter is not very good.
The best free OCR-converter is "OCR Terminal"(2):
But is has 3 disadvantages: Max. file size: 10MB, only 20 files free per month, registration necessary.
(1) Free File Converter
(2) OCR Terminal