Edit a text smart object layer added using PLACE...

If you are planning to add your signature to many picture files now and in the future, consider making a signature/copyright brush which you can save and use subsequently. Once you have the brush, it is applied with a left click, uses any color that you desire, and the size can be adjusted within limits. It's very versatile.

Here is a link for your reference.

A Watermark Brush is Easy to Make with Photoshop Elements

If you google this topic, you will get many hits, including you-tube assisted work-flow.

I suppose your pdf signature is coming from a 'text' layer (a vector layer), not from a bitmap (scanned) layer. The background is transparent (checker board display) and you have saved it in a suitable format like psd, tiff or png to save transparency.

When you 'place' the signature on your document (or drag it from the photobin), a smart layer is created.

That smart layer can be moved, enlarged, tilted etc without losing quality. You can add and group adjustment or fill layers to change color.

But you can't edit the text itself from that 'smart layer'. Either you edit the text in the signature psd and place it again, or...

I would open at the same time the signature and the photo files. From the signature file layers panel, I would click and drag the text layer to the photo file shown on the display. The result is not a 'smart layer', it's a real text layer in which you can edit the text. There is no link to the original signature file.

If you have to apply the signature to many files, simply keep the signature file open.  I prefer to set the preference display option to 'allow floating documents in expert mode';  that makes moving the text layer easier from the layer panel of the signature file.