Guide
Before you upload a PDF to an online converter
Updated
This is a thirty second decision that nobody makes, because the upload button is right there and the question does not present itself until afterwards.
What the upload is
If the file contains personal data and you upload it to a service to convert, that service is processing personal data on your behalf. That makes it a processor and you the controller.
UK GDPR Article 28 requires processing by a processor to be governed by a contract that is binding on the processor and sets out the subject matter and duration of the processing, its nature and purpose, the type of personal data, the categories of data subjects, and the obligations and rights of the controller.
Terms of service are not that contract
Clicking through terms on a free site is not the same as having an Article 28 contract in place, and it is generally not written to be one.
Nor does it usually answer the practical questions: where the file is stored, for how long, who can see it, and whether it is used for anything else.
The simpler answer
Do it locally. Desktop applications and command line tools do this job well, offline, on a machine you already control, and the spreadsheet you are converting into is on that machine anyway.
For an occasional single file this is not slower than uploading, and it removes the question rather than answering it.
Where a service is genuinely needed
At volume, or where recognition quality matters, a service may be the right answer. Use one your organisation already has a data processing agreement with, which is usually one it already pays for.
The test to apply is simple: if you would not email this file to a stranger, do not upload it to one. It is the same act with a friendlier interface.